Because they have serious debt problems and are having to sell players.
http://www.guardian.co.uk/football/2010/apr/27/jimmy-bullard-hull-city
"Hull, who have commissioned an independent audit of their finances, are £35m in debt, in effect relegated from the Premier League and trying to stave off the threat of administration or the need to enter a Company Voluntary Arrangement. With the atmosphere between the club's owner Russell Bartlett and the chairman Adam Pearson extremely tense, it is not impossible Pearson could resign. For the moment, he is working to reduce the player wage bill from £40m a year to £15m while also helping restructure the debt by re-negotiating loans with various creditors."