Couldn't be something so common as the wind, could it?
Yeah, Mars has lots of little dust devils...they could have blown the dust away.
Wonder why NASA just didn't add a little mechanical brush above the cell panel and use it every so often. Weight vs. cell efficiency...I'd bet they could've come up with some kind of lightweight system. Oh, well. Maybe next time.
I am thinking the wind plus lack of static electricity, it just blows right off.
If you look at any of the moon landings with the moon rover, it is always unusual to see how the moon dust cascades off the tires without billowing clouds.