"Butt sinking in". By this do you mean your pelvis curls underneath you causing lower back to round. I.e.: You go 'duck butt'.
If that is the case, then the cause is most likely due to lack of hamstring flexibility and weak spinal erectors and local core muscles.
If, as is quite common, as you descend your lower back excessively arches, and then at the bottom your pelvis curls underneath, then you have lower cross syndrome: tight hip flexors and hamstrings, weak glutes and abdominals.
Br