That is an actual thing that happens, yes. What happens is that for whatever reason at a facepull or at the very initial moments before the monster 'reassesses' it's enmity priority at the start of the fight it will rather randomly pick a player to aggro on.
Sometimes the monster won't have time to even get near the target before it sees the tank has highest enmity and will just turn around and go towards it. Aggro shift isn't instant I do believe enemies reassess every few seconds.