Your point is understood but is selling 2 billion dollars worth of shares sound normal? That's a lot of money.
Compared to his total shareholdings it's a large amount, but certainly not unheard of. He's selling off less than 20% of his shares as far as I can tell.
Note that I'm not saying it's not possible that he's "lost faith" - could very well be. Just saying you can't read too much into sales like this, as anyone sane will diversify if they have billions locked up in shares of a single company.
Now, if there's a wider trend of sales amongst Microsoft management (I don't know either way - I don't pay much attention to MS), then it's worth taking notice.
