Why? If you have to ask, you apparently don't live in Orange County, Calif!
It's a tradition that started -- on a bet -- 29 years ago and has been a growing event ever since.
Residents call it the Mooning of Amtrak, but now there is concern that last year's moon pie, uh, event got outta hand. Reports of public urination, nudity, sex, mischief and general mayhem accompanied the normally staid pants dropping festival.
A town ordinance is being considered that would block the area in Laguna Niguel where the mooning takes place.
Update: The Laguna Niquel council stayed off mooning in specific, but expressed disapproval for public urination and other rowdy conduct.
Here's the story and video: