Over-spool = too much boost = too much pressure = pistons out side of block, or melt or it all will just go boom.
But de-catting as being spoken about in this will increase the likelyhood of over-spooling, as will the various other mods being performed on these guys hotted up subies.
many of them run their own boost controllers, so it bypasses the factory boost control often to allow a higher boost, which is fine, If the cat is there, but de-catting creates more flow, therefore less ressistance on the turboo resulting in faster spooling, therefore increasing the likelihood of over-spooling. and without these factory boost-cuts in place engine go boom-boom. so if you have a standard system, you are less-likely to go blow, but with average boy-racer mods in place, chances are somethings gonna give, and it's not gonna be something cheap.
edit:amiright?