Generally I beleive it's a hamstring flexibility issue... at least that's what it was for me. Your hamstrings reach their flexibility limit and then they start pulling on your glutes and thus your hips have to "round under" or "butt wink" as it's called.

Try hamstring flexibility movements on your off days. Also try very light weight squats while focusing on only going as deep as you can before your lower back round. Focus on staying tight in the low back and use this to stretch the hamstrings (or whatever muscles are holding you back).

Hope this helps,

IW