That's right, dfb. When a wheel is powered with normal, forward tread pattern, and as it starts to slip the angle of the tread is such that it provides maximum traction and also keeps the treads as cleans as possible by pushing the dirt or mud to the outsides of the tread. As we all know, once your tread fills up with mud, you're good and stuck. The tread pattern is always wanting to be pushed into the soil V first, if that makes sense.
When the wheel is not being driven, slip is going to be manifested by the tired spinning slower than ground speed, and the same thing happens, but this time it's the ground pushing on the tire instead of the tire pushing against the ground (active force anyway). Same pattern, same idea. The V part of the tread pushes in the the dirt first.
Pivots typically have opposing tread patterns on each tower just so at least one wheel has maximum traction in each direction, though I have a few towers here and there where the tread is not this way, and it seems to not get stuck going in the wrong direction. And I haven't noticed any wear issues.