I have had that a couple of times on the Breathe Free II's. The plastic on the side of the heel cushion just broke, and after that the cushioning did not work any longer.
I returned the shoes to the local rep, and got new ones. They just asked whether I had stored them on a floor with floor-heating, but that was not the case. Probably they may dry up if stored on a heated surface, which might explain the cracking.