A few amateur thoughts…
A pressurized cabin is exerting pressure outwards. The door opened outwards. People say the pressure differential will somehow prevent opening. I doubt that. The door has seals that require the closing mechanism to overcome the seal pressure and are constantly used, so I doubt they are designed to present a lot of friction during operation. The added pressure at various altitudes are probably not hard to overcome. The door mechanism itself has leverage advantage designed in, to get a good seal.
As to overcoming the airflow. The airflow near the skin of the craft varies in position at various speeds and altitudes. It is not uniform along the length of the craft. Some aircraft have variable widths of fuselage to account for this and be optimal at certain speeds. Wasp waist for instance. It is possible that certain portions of the fuselage are actually in low pressure states at various times. The airflow being quite different than a laminar flow all along it.
My personal suspicion is that you can open doors on aircraft most of the time. Preventative measures will likely add complexity that might fail in emergency situations. So the best option is to go with the possibly false trope that it can’t be opened.
Had to listen to an annoying air leak all across the Pacific Ocean in a 747. The pressure differential did not seal it.
It is a bit of a misunderstanding of which doors are being talked about.
Pressurised aircraft passenger doors have used what is known as a plug door for quite some time. The door edge is bevelled, and the bevel is wider on the inside of the aircraft. Which by itself sounds impossible for an outward opening door. But with a clever design of the hinge mechanism, the door is opened, first inwards, tilted and then swings outwards on a partial diagonal, clearing the door frame. So these doors won’t open when the aircraft is pressurised, as higher pressure on the inside seals them tighter. The force over the area of the door during flight at altitude is impossible to overcome.
However emergency exit doors are not plug doors, nor are cargo doors. This incident applies to an emergency exit door, which would not have had the benefit of the intrinsic sealing of a plug door.
To add to my above post. Looking at pictures of the aircraft after the incident, it was a bigger emergency exit door that was opened - I was assuming it was an over wing exit. But the exit used looks to be of the same size as a conventional passenger door - so probably is a plug design. The general arrangement of the door and hinges look that way. So yes, it would not have been possible to open when there was any amount of pressure differential involved.
Thanks for the information. Nice design idea.
Now I’m starting to doubt my memory about the over wing exits. I think I remember on 737 over wing exits the instructions are to pull the exit door inside and then throw it out. So they would be a plug as well. In general, a plug design makes the aircraft designer’s job a whole lot easier. (No doors at all would be ever better.)
I always wondered how that worked. Thanks. I figured it was in the hinges… somehow.
The very older Boeings (and very early Airbuses) that have overwing exits used to be as you say. That’d be 707s, 727s, early 737s, those 767s which had overwing exits. For them it’s a plug door as you say. When you activate the deployment handle the door unlatches and from its weight simply falls inwards into your lap. It weighs about 40-50 lbs. Then you set the hatch on the seat or turn it on edge and throw it out the exit opening.
Starting in about 1990 or so the later model 737s and the later Airbuses have changed that. The overwing exits now are outward opening, hinged at the top, and include a powerful spring to flip them open after you pull the deployment handle. Once they flip up, they latch in that position.
Because they open outwards, cabin pressure does not provide any safeguard against accidental or malicious opening. So they’re equipped with an electrical solenoid deadbolt that locks them closed during takeoff, flight, and the early part of landing. But which are unlocked at the gate, while taxiing, and in any sort of situation where an evacuation might be a good idea.
What if there’s an aborted takeoff or crash landing that requires evacuation? I’m sure the designers have considered all these contingencies, but superficially it seems like there’s some non-zero chance these exits might be locked when you need to use them. 
The deadbolts are spring loaded to retract. It takes electrical power to hold them engaged. There are several parameters like engine power, aircraft speed, etc., required to engage them. Anything that looks like stopping releases the locks. And in a no-kidding crack-up, once the electricity stops, they also spring open.
I’m not an expert of the FAA’s requirements for these designs, but it’s all in accordance with their idea of safety both in normal ops and during / after a crash or crunch.
That sounds much more important than preventing rare bizarre incidents like this that didn’t hurt anyone. I’m afraid a design change to address this incident might itself have serious safety risk. Yes, the change would be carefully tested, but there is always risk of introducing a new flaw.
Ah. Thank goodness some of the memory cells are not failing. I was struggling to reconcile the faint memory with recollections of current reality. 
Back in a previous life I did a lot of flying on 737s of that era.
This sounds exactly like the kind of interlocks I was envisioning upthread:
Nice summary video posted on the blancoliro YouTube channel.
Interestingly the door is termed a semi-plug door. Some nice bits of video of the mechanism.
There are still plenty of aircraft flying around with the pull in and throw out style exits. Our A320 ceos have those while the neos have the newer spring-up ones. The oldest ceo in the fleet is a bit over 11 years old.
For a start he seems completely ignorant of the many, many, many military aircraft throughout history that all flew faster than 150 knots but still had hatches, canopies, guns, gun turrets etc. that opened into and against outside airflow. Here is a pic of a Fairey Battle with its rear gunner canopy in the open position:
Fairey Battle
And there are tons of examples from the 1930s of planes with similar arrangements.
IANA aircraft door designer nor physicist; I have a good memory & have seen many episodes of Air Crash Investigation, highly recommend. Once, they showed a crash caused by human error caused by mechanical error. So, I don’t see it in the story here, but maybe a Final Destination-like cascade/combo of factors from the show: the door is faulty - it locks almost all the way, but not completely. An indicator light or warning message to be triggered for the pilots fails due to a cheap blown fuse or frayed wire. 1 pilot flies, the other checks instruments and runs down checklists but the one who’s supposed to check on the door lock is distracted. Everything looks OK then the plane hits a pocket of turbulence, which because the door ain’t totally sealed puts tremendous pressure on the rods holding the door in place, it opens. OR like happened in 1 famous crash, the bolts used turned out to be counterfeit, not rated at the strength they said they were. Analogous to software bugs in a process more likely in a program or app cascade, where 1 calls another which calls another, anything mechanical like a door handle got moving parts; anything with moving parts can break.
I dunno for sure that’s what happened, but something in my mind got tripped seeing this incident, most likely covered in 1 of the Air Crash Investigation episodes.
As I understand it, Airbus helped out here. Unlike Boeing, Airbus designed a helpful hydraulic door opening assist to get the door opened quickly when needed. Turns out it helps when not needed as well… Boing crews never see this feature and don’t understand. At least that is what the Boeing and Airbus pilot in the story said.
Some Boeing doors have a boost system, be it pneumatic, spring, or hydraulic to shove (or help shove) partly open doors open the rest of the way. The narrow-body 737 doesn’t have that feature but (some of ) the bigger doors of the larger planes do.
As to Airbus, I know I personally don’t know. The pilot manual for A320s is very short on mechanical details but does say that each door is equipped with a booster that assists in door opening in an emergency.
A nitrogen charge from a dedicated ( one for each door ) reservoir supplies pressure to an actuator which forces open the door when the door is unlatched to open…and the escape system is armed.