Yeah, low psi could be the culprit. The manual states over 20psi if I remember correctly. But I usually run 16psi on the dirty parts as a compromise. My MT21 front did the same thing. For some reason that single knob doesn't wear much. D606s wear a bit smoother. However pavement screws up knobs for sure. I also ripped bits of knobs off a MT21 rear in Death Valley. That place has lots of rocks and I managed to get bogged down in deep loose rocks a time or two. Riding out of them was a futile experience. I think that's what did in the tire.