Justin: Knees Coming In During Squat

Did football lifts today. Only big lift was

Squat:
185x8
225x6
275x4
315x3

Probably could have gone a little harder on these if it wasn't for the short rest and supersets with other stuff.

One thing I had a problem with though- knees coming in starting from about 1/4 the way from the bottom. I came out of the hole pretty good, knees came in, I pushed them back out, and did the rest of the lift. It's kind of been a recurring issue and I don't really know why or what it means...