Six months after the accident, the exact chemical reaction that caused the drum to burst is still not understood. Indeed, the Energy Department has been unable to precisely identify the chemical composition of the waste in the drum, a serious error in a handling process that requires careful documentation and approval of every substance packaged for a nuclear dump.
Given the meticulousness in segregating and/or inerting reactive compounds in regular hazardous waste along with it's commensurate documentation requirements I find it stretching credulity that anything other than someone purposely placing something like a strong oxidizer in a container calculated to erode over a period of time as the cause of this.
I have encountered a situation where an oxidizer was accidentally contained within a drum which turned out to have an absorbent that had carbon content significant enough to react (The personal protective tyveks and rubber gloves are probably still in the rafters there...) but I can't imagine that everything that could conceivably go into a drum containing plutonium waste would not have been checked in triplicate and modeled/simulated til the cows came home.
Then again, governmental/contractor stupidity does appear to have gone asymptotic these days.
Asymptotic implies that there is a limit to the stupidity.