Actually,, rolling onto thier backs and showing thier tummies is a sign of submission,,, I dont think it was a trigger to the other dog attacking,,, Most dogs know it is a signal of a lack of interest to fight and an acknowledgement the other dog is Alpha....

the other dog just was in a mood and wanted a fight,,, not a nice doggie,,, owner needs a good talking to,, perhaps presenting them with a vet bill might be in order!!!