I would say a lack of tire pressue and pie boy riding style leads to that.
I would say thats what happens when you dont ride like a fanny ;)
Ive got 819's, love em. Im pretty skinny but like to blat through rocky trails (the number of dents in the rear wheel show that) and have ridden dirt jumps, 4x tracks, downhill tracks all without worry :D I highly recommend them.