What is the main cause of piriformis syndrome?

Irritation of muscle. Piriformis syndrome is caused when the piriformis muscle compresses the sciatic nerve. Since piriformis syndrome is usually caused by sports or movement that repeatedly stresses the piriformis muscle, such as running or lunging, prevention is often related to good form. Avoid running or exercising on hills or uneven surfaces. Warm up properly before activity and increase intensity gradually.
Refer to answer. This is very rare. I have found no cause for this yet. However our experience points to inflammatory conditions.