My guess is turbulence
Thin air cools rapidly*. If you have temperature differentials, then you often get wind. As the wind blows over a crater, it might create turbulence that could remove the dust. That's my guess, anyway.
I don't know if the MERs carry wind guages. If they don't, it'd be interesting to put an instrument like this on future landers, assuming the technology would work in the very thin Martian atmosphere.
*I live with this everyday here in Colorado. When the sun goes down, the temperature drops 20-40 degrees quite rapidly.