“World’s richest person reduces Microsoft stake to 1.3 percent”
I believe he has diversified:
Not necessarily. He just has to believe that the return on his investment will be greater (financially, in good will, in results) than the same money would be in Microsoft.
How is that different?
If he expects to get a greater return elsewhere, that's because the upside of Microsoft is limited enough, over the time horizon he's concerned about.
So what I was trying to express to OP is that while Microsoft's value may not sink (that is, now is not at the top of any curve), the value of the investment may be rising faster in various measures than the same money in Microsoft would be.
Another reason to sell is you believe there is a better investment elsewhere so you want to transfer the money there.
Another reason to sell is you want to use the money for something other then growing money. (buying a family heirloom with only sentimental value)
When an insider sells stock that is considered meaningless by investors because there is no way to know which of the above is the case. When an insider buys stock that means the insider believes the company is a good investment and so that means something, but selling means nothing.