I wonder if it was combustion (perhaps spontaneous) of the "organic materials," or a fermentation process in the "organic materials" which caused the containers to burst.
Either of which would have been helped along by the additional heat caused by the radioactive decay.
I also wonder what kind of bacterial or yeast mutations resulted from the radiation.
Germzilla?
Super wine-making yeast able to directly ferment grapes to 100 proof?
Inquiring minds want to know.
Especially about the latter. 
Terry, 230RN
Neither.
A lot of nuclear material processing involves acids, specifically nitric and sulfuric acids, which react with organic materials to release gas (CO2 and water vapor, as acids are oxidizers).
"Organic" kitty litter is typical absorptive organic refuse (think corn cob based particles like used in a tumbler)
Thus bad.
Regular "kitty litter" like they should use in this case is mainly benotite clay particles...non-reactive with acids being already oxidized. They also absorb massive amounts of water (dewatering acids completely makes them virtually non-reactive), they easily for colloidal suspensions (gets in all the nooks and crannies) and swell on absorption (sealing any leaks and creating impermeable films over materials). Basically, exactly what you want to fix and hold liquid bad stuff.
And nature provides it to us by the megaton!
A better question is WTF make "organic" kitty litter any more "organic" than kitty litter you dig up out of the ground? Or ironically, is the name because it has organic compounds? That would be hilarious.
But to your point about bacteria or yeast, well, there are actually some bacteria that can live INSIDE an operating nuclear reactor, but it wouldn't worry about mutation.
At those levels of radiation, successful mutation (small DNA errors that don't completely impede the functioning of the cell, because if it impedes reproduction, it doesn't go anywhere) is really tough, as they get clobbered so much that they either make it through a reproductive cycle with no errors that matter, or the cell ceases to function.
Also, given DNA's fragility, the bacteria that love at extremely high temperatures (eg ones in geothermal vents) are ironically more bad-ass. The thermal issues are actually harder to deal with biologically than the nuclear induced damage. Since a nuclear particle impact can alter single (or at most a few) bonds in a molecule, and DNA is somewhat error checking (due to 3 letters per codon and two strands), if one side of one rung of a DNA molecule is blasted, it typically (IIRC) results in one side being a non-coding/non-useful strand and the other being fine. Since the two strands are data-mirrors of each other, and only one side codes to mRNA, there seems to be some robustness there, as likely only one side would be hit, but both sides can be used for replication of the DNA in cell division.
I don't remember if ribosomes can "error check" (I.e. Does the step to the next codon require a compatible amino/tRNA there before hand? Or are there codons that are the equivalent of a NOP), but it seems likely.
Thus some damage can occur without resulting in an inability to mfg that protein, or in the case of a mutated protein, a non-functioning or alternate functioning one.
Dammit, now I'm stuck in a wiki-walk about DNA damage and protein manufacture.
