Sounds like your pony both cut throttle and went to maneuvering flaps and beyond. It's one of the great advantages of the p51 over the La7, that it has the ability to release it's speed quickly and get into a good fight position early. I do that sort of thing all the time, so am not surprised at all that it happened.
Also, the Mustang is well suited for diving. It can rip it's wings off, but if you can minimize the wing stress in turns, you can stay in control at very high speeds. Sounds like you ran into a good pony driver though.