Besides osteonecrosis, what other mechanisms may enable a dislocation to cause arthritis?
Recall this picture: dislocation can be associated with an impaction injury to the joint surface, and torn or stretched ligaments
The MRI of the knee above shows a so called "kissing contusion"---the two high signal areas were impacted against each other. Although the MRI does not show the articular surface very well, we can infer from the appearance of the bone that there was also an impaction injury to the cartilage, with possible irreparable damage inflicted.
Further, if the ligaments heal loosely, if at all, the joint may be unstable, and an unstable joint may be subject to repetitive "micro-trauma", as the talus (in the example above) rattles around in the joint. (This is the same mechanism by which a loose lug nut on your car may cause your tire to wear out prematurely.) Because of instability the forces in the joint are not applied across a broad area, but rather focally. And because P=F/A, a smaller area for a given force results in higher pressure.