Yeah, I'm not sure exactly HOW the tendons crystallize, though. It may be directly influenced by anabolic steroid injections or indirectly influenced by the "benefits" steroid users receive (quicker muscle recovery resulting in more frequent exercise). There is a debate among doctors over what actually causes the crystallization, the long held belief being it is a form of acidosis brought on by excess lactic acid* produced during exercise (Wikipedia is a hell of a tool, BTW). This would seem to make sense: exercise causes excess lactate to build up and crystallize, with rest allowing for the excess lactate to be removed; frequent exercising of the same muscle group/tendons, though, does not allow for the quick removal of the acid, resulting in increased risk of injury.
This follows in Prior's case, with his leg muscles being abnormally large, playing during the steroid era, frequent tendon issues in his legs, etc.
*for more on lactic acidosis, consult the movie A Few Good Men