It appears that there are two primary candidates for this failure; one is that the thrusters were not properly tested and approved for ignition after being exposed to death pressure and its consequent near-zero temperatures. As a result, the space probe would have failed to ignite its engines at the proper time, and dropped 1500 feet or so (I believe) to the Martian surface.

The other possibility is a sequence of events which I believe has been duplicated in test conditions. Namely, the descent thrusters were to be turned off at landing by the hyper-extension of the landing legs (due to the probe settling on them) tripping a shutoff switch. Apparently, though, the initial shock of deployment as the legs swung down out of storage was enough to trip the switch and shut off the engines mere seconds after they'd fired.