ha ha --- l'm willing to bet, that's how it ended in this photo --- one or both of them got the speed wobbles and they got flipped.
l should know, buddy of mine tried the very same thing in a parking lot maybe years ago. he was doing fine for a while but soon as it reached a certain speed, the wheels started to shimmy really hard, in a split second, my buds was ass-over-tea-kettle, rolling/sliding with a 50lbs shopping cart. it was funny as shit, my stomach still hurts to this day from laughing so much.
it was in the dead of winter so he was all bundled up so he didn't get all too banged up -- holy crap, if he crashed like these guys are dressed -- two words ----- hospital trip
My friend, with lot lucky everything worked fine. almost happened exactly the same thing that happened to his friend, narrowly missed.
So, it really is very risky practice this sport... going to the hospital is almost certain ! hahahahaaa...