Why or how, assuming correct storage and use do condoms fail?
As far as use I have to believe any adult human from pretty much any time could be shown a condom and told what it is intended for and they could figure out exactly how to use it.
How do condoms break or burst? Assuming they are used correctly, and not by Clark Kent,
I’ve had it happen 3 times. Once many years ago with one brand I’ve forgotten and two times with Durex featherlite. In all cases they where applied correctly and broke during normal usage. The durex are pretty thin and my SO and I changed brands to the swedish rfsu, after no. 2 broke and it didn’t happen again while we still used condoms.
I wish I could claim it’s my superhero equipment’s fault, but I doubt it 
The parents don’t get involved. They expect the factory to do everything.
They can “fail” by slipping off. When you put on a condom it should be a tight fit so it generally stays on even during vigorous use. Once the condom has served its purpose the tightness can, shall we say, shrink. If you are not careful the whole thing can slip off and spill its contents in unwanted places.
Well, the parents can’t do everything, the condom has to want to succeed.
If the plot of Your Sister’s Sister may be taken to represent plausible scenarios,
some condom failures are the result of deliberate sabotage, by a woman who desperately wants to have a baby.
I know, not quite in the spirit of the OP’s assumption of “correct storage and use”. I’ll go hang out in Cafe Society now.
They just rip sometimes while in use. I don’t know why it happens because they seem pretty strong but I have had it happen several times.
Need answer fast? 
Sometimes they break, my son is exhibit A.
Sometimes they slip off, leading to a certain amount of amusement, then panic when you realize you can’t get the damn thing out of there.
Just another note from decades ago - if she says “it’s not going inside, but just in case it gets close, wear one”… Condoms tear easily if rubbed for a short while on sticky or dry skin rather than very wet and lubricated skin. Just sayin’…
Conversely, if they’re undersized they can rupture during the act.
Heh actually I was curious since currently condoms are my sole birth control method about the 2%? failure rate. Which seemed baffling, I’ve personally never seen a condom bust or break(just lucky?) in intended use.
Are condoms not marked with an expiration date? I suppose condoms could tear more easily if they are old, but I am not sure if they have a -use by this date- on the package.
They do.
A condom rupturing happened a number of times to me. I don’t know why because I noticed in similar threads that there many people like you to whom it never happened. Maybe I’m doing something wrong, but if it’s so I’ve no clue what. On a couple occasions, I didn’t even notice immediately it had ruptured.
And as mentioned above, indeed, one shouldn’t stay “inside” while the penis is becoming flaccid, or else the condom is likely to slip when withdrawing.
However, telling a (potential) partner that “we have to, tonight, because this is the last day for the condom use-by date” does not seem to be very persuasive. 
The OP also says to assume proper storage, but in real life, a lot of condoms are stored improperly. The cliched rubber in the wallet not only ruins the leather, it can ruin the latex, too.
I’d suggest the OP seriously consider a second form of birth control to be used in addition to the condom.
–Cliffy
And of course there’s the imperfect use - not using condoms during frottage, for example, or not immediately withdrawing and disposing of a condom. (Say, if you linger and get soft while still inside a woman the seal is no longer tight, etc.)
Water based lubricants are safe to use with condoms. Petroleum jelly is not. And by “Not safe”, I mean “Will fail”.
Science experiment. Blow up condom like a balloon. Rub petroleum jelly on it for 5-10 seconds. Pop!
Fortunately I had one fail on me in 1978. I say fortunately as this breakage lead to the birth of our third child, a daughter who has now given us two delightful granddaughters. I just do not know where I would be today if the faulty import from the East had not torn during normal use.
Needless to say after the birth of the unplanned daughter I rushed off to get the snip. Far safer. 