Honestly I don't know what it is, but it could be that you're putting too much sudden stress on them. How frequently do you sprint? If you're just starting interval training, I'd say once or twice per week. Younger athletes can get away with 3 times a week because of faster recovery rates. I'm assuimg you're under 30 though, and in good shape (not to say people over that age can't do it more weekly, just generalizing).
Rest your knees, give them time to adjust. Do it even once a week at first to give them a break, away from heavy squat days if possible, and walk on other days just to keep your joints flowing loose with blood (if that makes sense, think AR!) so they don't get sore or cramp at all. That's basically what I think... hope it helps!