There is certainly some limit to how much of his stake he could sell out of at once without spooking the market and destroying retail investor confidence, though it’s hard to say what that limit is. Without some cover story I suspect an $8.5 billion sale would be newsworthy and significantly impact the share price.
Arguably the Twitter poll and attendant media circus with Bernie Sanders about taxation was also to provide pretext for a large sale without spooking retail investor confidence.
He needed to liquidate stock to pay taxes from previous stock options he'd been given either way of the Twitter poll. I find a lot of the comments here to be pretty off-base and ill-informed to say the least.
