Humor looses much flavor when an explanation is needed, but since you have asked, I cannot in good conscience deny you a straight answer.

Mr. Ledger starred in a movie, Brokeback Mountain, that was essentially a homoerotic romance. The same man, this time the actor, vice the homosexual character, Ennis Del Mar, was found dead, naked and face down. This position, while not exclusive to homosexual intercourse, or homosexuals, themselves, frequently comes to mind when homosexuals are mentioned. Since Mr. Ledger was not a homosexual, that he should die in a manner similar to a position that Ennis might have taken is unexpected. The unexpected is the very essence of irony, both verbal and cosmic.