Genetics.
I don't know if mine were technically considered 're-constructive' or not....but close enough. Me knees started dislocating quite easily around 8th grade...everytime it happened it landed me in the emergency room, and a several week recovery. And each time it happened it made it easier for it to happen again, because the tendons and stuff were getting so stretched and just messed up in general. I had two surgeries on one (the first didn't work...did NOT stay with that doctor, as he just wanted to do the same thing again), and one on the other. I still have screws in one knee.