Not knowing anything about Hinton's work, I am guessing there is no mystery to why he left. Many people leave after a couple of years. His initial grant of RSUs has vested and he wasn't able to make a sufficiently large impact within the company to justify him staying.
The norm is a 4 year vesting period - but if you are doing important work and having a big impact, you'll be given more grants over time. Those will then come with a new vesting period. This is a very normal way for Silicon Valley companies to retain their engineering talent.
