This little muscle can cause a lot of problems. If you google "wiki piriformis syndrome" you'll find out that it can cause very painful sciatica (which is literally a pain in the butt or down the leg):
Inactive gluteal muscles also facilitate development of the syndrome. These are important in both hip extension and in aiding the piriformis in external rotation of the femur. A major cause for inactive gluteals is unwanted reciprocal inhibition from overactive hip flexors (psoas major, iliacus, and rectus femoris). This imbalance usually occurs where the hip flexors have been trained to be too short and tight, such as when someone sits with hips flexed, as in sitting all day at work. This deprives the gluteals of activation, and the synergists to the gluteals (hamstrings,adductor magnus, and piriformis) then have to perform extra roles they were not designed to do. Resulting hypertrophy of the piriformis then produces the typical symptoms.
Apparently in some people (16%?) the sciatic nerve goes through this muscle rather than under it and it is thought this can lead to a propensity toward the syndrome.
Seated activities such as biking and rowing can cause problems:
When not balanced by lateral movement of the legs, repeated forward movements can lead to disproportionately weak hip abductors and tight adductors. Thus, disproportionately weak hip abductors/gluteus medius muscles, combined with very tight adductor muscles, can cause the piriformis muscle to shorten and severely contract. Upon a 40% increase in piriformis size, sciatic nerve impingement is inevitable. This means the abductors on the outside cannot work properly and strain is put on the piriformis.
Sigh. Every little muscle in the core can cause such problems!