Originally Posted by BendtheBar
Before we start trying to fix this, I think it's best to look at the big picture to see if there other issues that may need to be fixed.
Do you have a video up of your squats?
There are some common squat form flaws that might be forcing your lower back to round in the hold.
Looking forward to this answer . . .
I too have this issue . . .My Physio spotted it. He tells me my hamstrings, flexors etc are more than flexible enough.
Can't figure it out either. Funnily enough I can box squat fine . . .