According to the reference below, hypertrophy was observed in the piriformis and obturator internus muscles in people with the piriformis syndrome. In addition, it was confirmed through endoscopy that the sciatic nerve touches the obturator internus muscle when pressing the piriformis muscle.

This can be interpreted that if the piriformis muscle is continuously stretched or overloaded due to body imbalance, incorrect posture, or muscle function decline, the muscle can gradually enlarge, eventually touching on the sciatic nerve right next to it, resulting in piriformis syndrome.

Moreover, the piriformis and gluteus medius (side hip) share the function of hip external rotation. However, if the side hip muscle gets weak, which is often the case in many people, the piriformis starts to compensate, resulting in overload. Thus, relieving this overload is key in addressing Piriformis syndrome.

References

Boyajian-O'Neill, L. A., McClain, R. L., Coleman, M. K., & Thomas, P. P. (2008). Diagnosis and management of piriformis syndrome: an osteopathic approach. The Journal of the American Osteopathic Association, 108(11), 657-664.

Hopayian, K., Song, F., Riera, R., & Sambandan, S. (2010). The clinical features of the piriformis syndrome: a systematic review. European Spine Journal, 19(12), 2095-2109.

Cook, J. L., & Purdam, C. (2012). Is compressive load a factor in the development of tendinopathy?. Br J Sports Med, 46(3), 163-168.