The stories mentionthat two Reservists with prison guard experience (one is Sergeant Chip Frederick, can't remember the other) were giving particular authority in overseeing the their fellow soldiers.
US prisons are violent places, where where abuse, rape, and violent acts are commonplace and are often perpetrated by the guards. From the 2001 Human Rights Watch report:
Guard violence, if not endemic, is more than sporadic in many penal facilities. In 1999, for example, news stories detailed a series of horrific stories of guard abuse--stories of inmates being beaten with fists and batons, fired at unnecessarily with shotguns or stunned with electronic devices, slammed face first onto concrete floors, and even raped by correctional officers.(57) In some instances, entire state prison systems are found to be pervaded with abuse. A March 1999 federal court decision concluded, for example, that the frequency of "wholly unnecessary physical aggression" perpetrated by guards in Texas prisons reflected a "culture of sadistic and malicious violence" found there.(58)It's even a cultural joke - go to jail, become a black man's "bitch" and be raped on a regular basis.
Given the condition of US prisons, and the culture of abuse that surrounds the idea of prison even outside them amongst the mainstream US, is it really all that surprising that the Reservists treated the Iraqis so savagely?
Add the pervasive anti-Arab racism that abounds these days (recall the "sand nigger" phrase that caught on in Gulf War I), the 'evil-doers' rhetoric from BushCo on down, and the extrajudicial prison oversight by mercenaries and the CIA, and this was almost inevitable.
We reap what we sow. Link to HRW's report,
No Escape: Male Rape in US Prisons.