Hedge-fund billionaires were already struggling to keep investors from heading out the door. Then along came another problem: big tax bills. David Einhorn, John Paulson, Steve Cohen and other high-profile managers cumulatively owed billions of dollars to federal, state and local governments for taxes, thanks to a 2008 rule change tied to offshore holdings that gave them a decade to comply. To make the payments, they had to pull some of their own money from their funds. For some, the bill came at a time assets were already shrinking.