There are a couple of possibilities:
1) That you picked up an object and it let all the air out of the tire. It doesn't take a very big object to do that - certainly one small enough that you wouldn't see it.
2) 72 psi to 81 psi = 9 psi = 13% pressure buildup. That's too much. That indicates overload/underinflation/too much speed. Is that typical of the pressure buildup?
To try to sort out what is going on, we need a better description of the failure. Did the tire lose pressure? Was the tread area still attached? What about the sidewalls? Photos would be helpful!