It's most likely the DFCO (deceleration fuel cutoff) ... the computer will cut the fuel to the engine when it determines it's not needed no matter what gear it's in.

 

Long mountain descents can really heat up brakes. That's why there are run-away truck ramps in the mountains. It affects cars too. There is a brake-check station on Pikes Peak where all traffic is stopped so they can measure the temperature of your wheels with an infrared thermometer gun. If your wheels are too hot you have to park until they cool off.

 

Why worn cause u don't know how to blip and rev match? I downshift my MT and DSG car all da time. Never had one issue over da course of 30+ yrs driving.. and I don't baby by cars either..