Well way back Fox bought the rights to make Watchmen as a movie, but they have pretty much been sitting on the project ever since. So WB comes along and makes a deal with DC to make Watchmen and Fox doesn't saying anything so they go ahead with it. Then after the movie is in the can, Fox suddenly starts yelling and screaming about having the rights. Pretty sure Fox waited till they were done cause they knew that they'd be more likely to get a cut of the profits without doing any work this way, where as had they brought it up before the film went into Production WB would have just scrapped it and Fox wouldn't have seen a dime.