One of the biggest predictors of back pain is restrictions in ankle joint range of motion (ROM), particularly ankle dorsiflexion (http://www.ncbi.nlm.nih.gov/pubmed/19674683). Lack of dorsiflexion, due to ankle impingement, limits your squat depth which is needed to lower down to the floor with proper mechanics. When you squat down your knee needs to translate forward relative to the ankle joint without the heel coming off the floor. This maintains a forward center of mass/balance. To test this, try squatting with your heels propped up and see how it makes it easier to perform a deeper squat.
Here, we demonstrate an ankle dorsiflexion mobilization to improve ROM. A common mistake is to put the mobilization belt or band too high. The band should be along the joint line, directly on the talus to provide stability as you drive the knee forward. This allows the tibia to glide anteriorly on the talus. The mobilization will reduce impingement at the front of the ankle and increase dorsiflexion. Evidence of its effectiveness can be seen here,http://www.ncbi.nlm.nih.gov/pubmed/16881463.
A key point is to elevate your ankle relative to the band which can be done by placing your foot on a step and anchoring the band below the height of the step.