If your low back rounds during a squat it can lead to many posture problems, such as back pain. Fix this muscle imbalance by doing these corrective exercises:
Myofascial release (e.g. foam roll) and static stretches for hamstrings, and adductors, and do an abdominal stretch on a ball or on the floor (pictures shown below).
Also, strengthening the Erector Spinae, Gluteus Maximus, and Hip Flexors will help.