Tight hamstrings and tight glutes can both do this, however.. I have pretty flexible hips and my back rounds every time unless I set my back. Here are some cues I have learned to use from other coaches.
Lift your chest up
Puff your chest out
Lift your ribcage up
Arch your lower back
Arch your upper back
You need to get the back set over emphasize thoracic extension, this way when you get down in the squat you will probably not have this over emphasized thoracic extension anymore, you will just have normal posture, but you really really have to lift that ribcage up, arch that upper back, because when you start to significantly tail tuck prematurely you are fucked, alot of people just keep lifting and adding more plates and thinking they are getting stronger, they are but not in the right way, they are not getting any drive from the hips,
they have lost the benefit (or much of it) of the hamstrings and glutes contributing to the drive upward, the stress has shifted to the quads (which is not good), squats are a hip exercise assisted by the quads not a "Im gonna do Quads today" exercise, (front squats diff story). Additionally, the lower back curls around under just like you said being pulled by your pelvis. The lower back is not meant to be very mobile it is meant to support.
Now your problem may very well be flexibility but it is not going to fix your squat, you have to work on what I said above to get it down pat.