You're losing your tightness at the bottom. I think this is because you're descending too fast. You're knocking yourself out of position at the bottom, possibly trying to feel for a bounce. Get yourself nice and tight before the rep shove your knees out and go down with more control. If your low back is locked in extension, you'll feel the bottom without trying and you'll get a real bounce off your hamstrings rather than a bounce off your calves.