Underneath the turbo is a device called the wastegate. The wastegate is what allows the turbo to "pin" by releasing boost at a particular level. The level is specified by an adjustabled rod, and alows you to increase or decrease the max boost level.

On the front of the wastegate is the actuator and actuator hose, and this hose comes up to connect to the pressure side turbo hose via a little copper right angle nipple. If that hose breaks or comes off either end you will lose your seal and the actuator will no longer be able to properly judge boost pressure; it will think it is at normal atmospheric pressure all teh time, evem when you are running at very high psi. Basically a detached wastegate hose allows you to overboost.

Check the hose to see if its rotted or broken off. Replace as necessary. Also check the wastegate to make sure the rod is in ploace and secure, etc.

-rt