Murphy’s law! Trust me, it’s real. 
One thing that happens sometimes is delayed school opening, which temporarily clears traffic a little as people figure out how to get the kids to school.
A common control scheme is that the time you must wait for a green light on the side road depends on how long it has been since the last green: If it’s been a long time, the wait will be short; otherwise, you must wait a long time while the main road is given priority.
That’s an easy one.
That other person wanted to send a text or dig something out of their glove compartment. So was hoping to get a red light to allow them to do that safely while they waited. So of course the light system granted them a green with no waiting. You on the other hand had no such desire to wait. So of course you had to wait.
See also: “Universe, Perversity Thereof”.
Only semi-joking here … I’ve taken to picking up my phone and threatening to send a text or take a note while driving just to make the lights stay green for my direction. It seems to work well. I suggest you try it. 
Some lights turn green only when a car comes to the intersection and, if it has been red a long time, it turns green immediately. There is a light like that a few blocks from me. I am on the main road and it is almost always green for me, since there are few cars on the other one. And the green in the other direction is quite short, unless a pedestrian has pushed the button, in which case it is longer.
I wonder if, after the long crawl to that point — where, if it’s kind of hilly or there’s a curve or whatever, you can maybe even see that it’s roughly where the snarl ends — folks tend to figure, ah, what the hell; I finally have a clear road in front of me, and I’ve already spent a significant amount of time slooowly getting here; how about, for a negligible amount of time, I keep going slow just long enough to get a good look at what all the fuss was about? Hmm. No, I see no apparent reason; I thought I maybe saw something, but on a second glance I didn’t. Well, time to return to regular speed.
And so does the next driver. And so does the next one. And…
Yes, we call them “smart lights” around here. You have to be careful because, even if your light just turns green and you think you’ve hit it perfectly, it often turns red again almost immediately if you aren’t close enough to register on its sensor.
Overall, they make traffic control more efficient. In Chicago proper, they have the old style lights which means that, at 3:00 am in the morning with virtually no cars on the road, there is no variance in their timing to accommodate that.