My experience is the opposite, I find that the buttock pain and leg pain which is the result of constriction of the sciatic nerve originates more often from interference from the piriformis muscle than from the spine.
The real question of course is why does the piriformis muscle spasm or collapse? For the same reason any muscle spasms or collapses - it is being overworked. In the case of the piriformis muscle it is taking over the jobs of the adductors: longus, magnus, and brevis. Their job is to adduct the leg, to hold the hip in it's socket. these muscles often fail due to weakness. Their direction of force and movement is medial or sideways and there is little we do that uses or exercises these muscles.
When they fail, the muscles in the buttock area pick up their jobs: the piriformis and the glutes take the job of keeping the hip in it's socket. Unfortunate,y they weren't designed to adduct the leg: they are too small and are quickly overwhelmed and collapse and spasm, pinching the sciatic nerve which runs right through the piriformis.
There is a three part solution.
1. Needle the motor point of the piriformis and any other reactive muscles in the buttock area. (I avoid using electro on any motor points because if the level of the current is too high, it's easy to burn or damage the nerves. I often use a low level electro treatment at a superficial level.)
2. Exercise the three core muscle groups: the adductors, glutes and abs. Lie on your back. To exercise the adductors, put a medium (softball) sized ball between the knees, squeeze the knees together, tighten the abs and form a bridge (10 seconds, 25 reps, 2 times a day). This exercise engages all three core muscle groups.
3. Make sure your magnesium, iodine, water, and electrolyte levels are where they should be. A deficiency of any of the four can cause cramps and spasms. The best magnesium I have found is "Dr's Best BRAIN Magnesium" (Magtein).