The prevailing theory about the Watergate break-in seems to be that Nixon and his men were paranoid/worried (pick one) about revelations involving Howard Hughes. Nixon had been embarrassed before about his connections with the eccentric billionaire, most notably in the 1962 California gubernatorial election. So, the assumption goes, the Watergate burglars were dispatched to wiretap the phones of Democratic honcho Larry O'Brien, who was a former Hughes aide and was therefore in a position to know a lot of Hughes-Nixon dirt.
I've never completely bought this bit of conventional wisdom. Maybe it's true but it seems to me that Richard Nixon didn't need a specific reason to spy on his enemies. The mere fact that they were his enemies was reason enough. In any case, one of the major articles on his relationship with Hughes and how it may have figured into Watergate ran in Playboy magazine in 1976 and it's available, probably for a limited time, on that publication's website. Here's the link and remember that if you click in the wrong place, you might catch a glimpse of a naked woman.