what could have caused this?

[old man roger]

Originally Posted by kawi636braaap

To my knowledge the clean air mod would only matter when the bike is near verticle and the oil can make its way through the airbox. Obviously during a burnout its not verticle and the oil should be circulating normally. If it hydrolocked its because of something else, maybe the oil seals are leaking on the valve guides or coolant from a blown headgasket. Maybe the oil pump went and you lost lubrication and the heat from friction blew the hole. Hard to say unless you rip down the motor.Does it still turn over?

I have seen plenty of rev limiter burn outs pump oil out the crankcase vent tube, but the fact he found a T shirt in the oil pan changes every thing

Some one has obviously done some work to that motor, if they were forgetful enough to leave a T shirt in the oil pan then they are probably forgetful enough to not properly torque the connecting rods..