Institutional owners like my pension provider that are legally obligated to maximize return on investments.
The other is hedge funds, in the case of CSX, it's Mantle Ridge. They make money, sometimes lots of it, by buying low and selling high. They move in on a company while knowing that the institutional owners will and/or have to support their plan to make the stock price go up quickly. They do that by having the company cut costs to the bone and beyond. Then they sell high and get out of town. They love EHH and others like him. They're just the modern version of a hired gun. Like the hired guns of the wild west, they leave the corpse behind and ride off into the sunset.