According to this article in The New York Times, most of the serious abuses at Abu Ghraib prison occurred on a single day in November.
The day of abuse — a Saturday — capped what had been the worst week for U.S. troops in Iraq since the March 2003 invasion. Nearly three dozen had been killed in a surge of attacks that left some other soldiers frustrated and frightened. Insurgents had attacked the Abu Ghraib prison and other U.S. bases in the area with mortars several times in previous weeks.
If that is truly the case, it knocks a body blow to the theory that this problem is a systemic one.