A Sante Fe paper has a longer update on the investigation into what led to the radiation release at WIPP. As had been reported earlier, it appears that the problems likely stemmed from Los Alamos personnel using organic cat litter as an absorbent in some barrels of waste. Organic material is much more reactive than the inorganic absorbents (such as clay-based cat litter) that they were supposed to use.
However, this was not the only problem, and it took a appalling series of mistakes by LANL workers to cause this to happen. The usage of organic absorbent was likely due to a typo in an updated LANL procedure, while LANL reported to WIPP that the contents of the barrel were nonreactive when in fact the mixture was highly reactive and potentially even explosive. Los Alamos should not have shipped the barrel at all and had WIPP personnel known the true contents of the barrel it would not have been accepted. Even after the accident, LANL reportedly did not inform WIPP of all the relevant facts of the case. State inspectors had not been allowed access to the LANL cleanup operation for several years, so there was also reduced outside oversight that could have contributed to what now seems like a lax safety culture at the lab.
The article claims that the problems at LANL may stem from a rush to finish cleanup in order to claim some funding tied to a meeting a deadline a few months after the accident ended up happening. Los Alamos has had a number of embarrassing problems over the years, so this just seems to be another one. One would expect that if all this really is true, the DoE will make some major changes to LANL leadership or at least the contractors running waste cleanup.