I wouldn't assume that there's some hidden reason for him leaving. First of all, leaving right before his stock is about to vest just means that he's leaving a little bit of stock on the table. As he said, the vast majority of his stock has already vested (as one would expect after 4 years), and he gets to keep that. It's the 5%, or whatever, of his stock that hasn't vested that he's losing.

I read this as "I've been working startup hours for way too long, and I'm tired of it, especially now that I have a kid I want to spend time with".