10% seems pretty high, running out of shares to sell of a company you own over the course of a decade? Definitely seems like you're hedging against a downturn.

10% is not a lot. Remember that the share price has gone up more than 50% in the past year. Assuming he wants to keep NVIDIA at a fixed percentage of his net worth, he would have to sell up to 33% of his shares (depending on the target percentage). Even after selling the dollar value of his remaining NVIDIA shares is still up $6 billion from a year ago ($8 billion to $14 billion).

Facebook too https://fintel.io/n/zuckerberg-mark

They know the market is going to crash soon-ish.

I'm not in the crash-soon camp.

It's just smart investing to take some profits at a historic high mark...