Could they be fighting? Is there maybe a hole in the fence...or dogs love to dig, maybe they have a hole dug under the fence and are scraping their backs as they go underneath?
I have 3 Beagles, In the past 8 months 2 have now been diagnosed with back Injuries. There kept in a double fenced in yard trauma seems unlikely. One is 2yr the other 9. Both dogs presented without fever. could it be some kind of bug/spider bite?