One theory is that it was a culmination of a long list of corruption scandals that, now he was no longer the majority leader, might have led to a Senate ethics investigation. The other theory is that he angered some of his mob connections, who beat the crap out of him.
Update: An op-ed at Bloomberg challenges (debunks?) the theory that the mob beat up Reid, and the media was too disinterested to look into the incident.
Update (4/3/2015): Powerline Blog was contacted by a man who claims that Harry Reid's brother, Larry, confessed to getting in a fight with Harry on New Year's Eve.