They were being programmed

During the screening phase of the experiment when Adrian's character goes into the chamber and watches a film filled with all types of imagery ranging from completely innocent to extremely violent, it is my opinion that he is at that point having a sort of program framework laid into his mind.

Later on during the movie, while Forest's character was speaking to the inmates, Adrian's character gets a little discombobulated and starts hearing echoes in his head. I believe it is at that point where his programming was somehow activated. I suppose he had been programmed to challenge authority and not back down nomatter what, and it was right after this point where he takes off his shirt, which angers Forest's character and begins the chain of events which escalated into the all-out battle scene.