Actually, most fan rubs are related to the gyroscopic forces at rotation - in fact that's a major structural design consideration for the fan. Obviously, gyroscopic forces on the fan are not limited to rotation - they occur during normal maneuvers and turbulence, but they are usually largest at rotation.
Obviously, things like abusing the T/R cutback speed, engine surges, and birdstrikes can also cause fan rubs - sometimes quite large. The fan rub strips are generally painted with a thin coat of a contrasting color so that new rubs are readily apparent. As McHale notes, so long as the magnitudes are within MM tolerances, it's no big deal.