He’s been with Meta for 11 years and is likely in a very comfortable financial position, given the substantial stock options he’s received over that time.
He also mentioned the arrival of a new child, and it’s well known that Meta's work-life balance isn’t always ideal.
On top of that, Meta, like many major tech companies, has been shifting its focus toward LLM-based AI, moving away from more traditional PyTorch use cases.
Considering all of this, it seems like a natural time for him to move on and pursue new, more exciting opportunities.