I believe that there is an anatomical variant in people concerning the piraformis muscle and the path of the sciatic nerve in the buttock area. Let me see if I can find that %...

yes, 10-30% is a figure often quoted:

http://en.wikipedia.org/wiki/Piriformis_syndrome

I personally think that sitting on stools, which are common in some large kitchens with islands in them, may also cause sciatica. I had a stool at my midnight job, and I'd perch on it doing paperwork (those were long painful 11 hr shifts), and I had a terrible sciatica after a year there. It took quite a while to heal up... and did so when I left that job (I quite midnights all together). Certainly an injury would do it too...but look around your life and see if you are sitting on something that is aggravating it, as well.