Everyone deals with crap at their workplace. Bad processes, unresponsive third-party dependencies, politics, etc. That happens everywhere, and anyone saying otherwise is either unaware or lying.
Eventually, there's a event that is enough standard deviations larger than the typical crap that the person just breaks. The veil of happiness is irrevocably pierced. And when that happens, all the other crap that they've been putting up with kind of comes crashing down. This is your "additional supporting reasons": they really existed the whole time, but they were at least tolerable before the veil was broken.
In my experience, it's rare that it's more like a dam that eventually breaks under pressure. There's usually that one event that finally breaks the person's will to continue. But then all the stuff that was being held back behind the dam is loosed and there's no coming back from that.