Blistering
This is what happens to a tire, or part of a tire, when it overheats. Excess heat can cause rubber to soften and break away in chunks from the body of the tire.
Too high cold inflation pressure or too much pressure build-up during use caused the tread to overheat. The heat is generated at the interface between the belt plies and the tread and the rubber melts, causing a local blistering.
More about Blistering and how to use tires read here.