The only thing I can think of, is that he/she has had to make a quick escape from somewhere and scraped his back along something - like a tree branch, or from diving under a fence where the gap wasnt quick big enough. If there's scabs, then something is healing.
Only other possibility I could think of (if he/she was my own cat), would be a skin irritation that's been scratched and scratched until the skin has been broken and is now healing. Skin irritation could be an allergy to something, or a flea irritation.
Scabs will eventually fall off and new skin appear underneath.
If there's any swelling or pain tho - pop him/her along to a vet in case kitty needs a course of antibiotics.
Only other possibility I could think of (if he/she was my own cat), would be a skin irritation that's been scratched and scratched until the skin has been broken and is now healing. Skin irritation could be an allergy to something, or a flea irritation.
Scabs will eventually fall off and new skin appear underneath.
If there's any swelling or pain tho - pop him/her along to a vet in case kitty needs a course of antibiotics.