Hey Humboldt,
Any chance it's overworked adductors? You started with heavy weight, progressed quickly while messing with form, and you seem to drive your hips forward more than up, which makes the quads work more and the hammies work less.
If your adductors were weak (most are) they may not have been prepared for the weight the rest of your body could handle.
Just a thought, you probably had strong quads and weak vastus, and overworked the weakest muscles.
Get someone else to check though, I'm on a layoff after hurting myself too, so might be exactly wrong.