I have a theory about this, but not one I'll likely ever be able to prove. Here's my theory:
We all know that the heads/sliders glide on a cushion of air generated by the spinning motion of the platters. Given the extremely smooth surface of the platters and now even extra structures to keep airflow steady they glide at a pretty stable height. But, what would happen if something becomes a bit distorted, such as an imbalanced platter, a warped slider arm, or even a foreign object such as a large dust particle gets lodged under the slider or it's arm? I imagine that it'll often begin to vibrate violently and repeatedly hit/drag across the platter surface. In the process causing more damage, thus more distortion of the air flow and even further vibration. Might only take a few seconds to destroy the platter surface entirely at 7200 RPM. I imagine that this also explains why certain models (specifically Seagate) seem to succeed at killing heads over and over again.
Just a guess really, but the best theory I've got.