Originally Posted by
JHG
A muscle strain is a stretching or tearing of the muscle fibers depending on the severity and can present with a popping sound with certain physical activities.
Based on your description of the location of the pop, I would surmise a strain of glute max, erector spinae aponeurosis or multifidus muscles – this comes with the assumption that your squatting technique is as prescribed by SSBBT.
A sacrotuberous ligament sprain is possible, but I would assume this would more likely be the cause in people performing ATG squats, which would also increase the likelihood of the aforementioned muscle strains.