Any force bearing on the operation of the gyro induces precession. Friction from uneven ruby bearings in the gyro gimbal mounts and pivot points is one such force; this force imparts precessive action on the gyro during routine operation, and it drifts. This is precession.
The single most common source of uneven gyro bearing wear? Sitting in the tie-downs on the ramp (or apron, if you will). As the gyro sits, not being moved, and the airplane rocks in the wind, the bearings wear unevenly, and this does more to damage the gyro than many hours of operation. Another source of damage is moving the airplane before the gyro has stopped spinning down. If the airplane is moved while the gyro is destabilizing after the airpalne has been shut down, it imposes significant forces on the bearings, which dramatically increase bearing wear and cause instrument damage.
In any event, the end result of such wear and damage is increased rate of precession.