I've heard this my whole life and continue to ponder just what got me in the left butt cheek and how that could have killed me! lol So, a sword to the rear? Sat on an evil bee? Wayward arrow?
What about a snake bite to the booty? I mean you could've been an adventurous explorer to the deepest parts of the jungle, far from any help and WHAM a deadly snake bites your booty. #itcanhappen
As for me, I don't have a birthmark. Maybe that means I just died of natural causes in my previous life?
I had a friend once whose birthmark was that his arms were "paint splotchy" when he spent too much time in the sun. Lol