What happens when 20 or more sims try to get through the same set of doors at the same time? They pile up and make things take way too long.
My sim is stuck in the middle of the crowd there, and she's very hungry right now (thus the red plumbbob above her). She's been stuck in this crowd for about three sim hours so far and it will probably take her another sim hour or two to get past everyone. There's not a whole lot that I can do other than reset my sim to get her out of there.
When sims bunch up like this, they try to take turns, but the algorithm they use is pretty bad and they all end up trying to step in front of each other all the time, which makes them stop and think about their position for a few minutes, which puts them in front of other sims that are trying to get by... Unfortunately, when the sim is at University, the Tuesday morning class is always at the student union building (which is seen here) and there are too many students that try to get in at once, so this pileup happens every sim week.