There's an efficient markets reason to believe this. Say you have some estimate of the value of all future earnings of some business, and then a group of peopl

There's an efficient markets reason to believe this. Say you have some estimate of the value of all future earnings of some business, and then a group of people divests and pushes down the share price. The future earnings haven't changed, so this pushes down the P/E ratio and makes the stock that much more attractive to buyers who aren't divesting. It's just not possible for it to have that much price impact.