It was probably the leftover crumbs tickling him. You can help him by wiping crumbs off with a paper towel dampened with warm water. If he doesn't scoot anymore then you're home free.
Sometimes, though, impacted anal glands make kitties scoot. Check his bum. If it looks swollen or red at the 4o'clock and 8 o'clock, it might be clogged anal glands. Ick. For that you'll need to see a vet.
Zoey did this once and I witnessed it, it freaked me out. I then realized she had a poop stuck on her butt that didnt make it in the litterbox. I think its from my hair cuz I shed a lot and she probly happens to eat it occasionally Only happened once though. That could be your case.