I just had a (contract) team member "quit" abruptly. Happy happy, oh I'm burned out, and need to take 3 months off, maybe more. I'm the last person in the world that would overwork an employee. In fact, I spend a good amount of my time pushing back against anything that would cause overwork. Burnout can happen for multiple reasons, and with various manifestations, so I may never know whether the person was actually burned out.

I've definitely been across situations where life intrudes into the work as well. Partners trying to get into a building (previous job) to confront an employee about cheating.

Humans are messy. Yes it might be the startup itself that has brought these problems on itself, but that isn't a certainty.