>>31640878This isn't new, it was present in the anime with Korrina's Lucario, Sycamore's Garchomp and in Super Mystery Dungeon.
If a Pokemon doesn't have a strong enough bond with its trainer (or no trainer like in Mystery Dungeon), it can't handle the power for long and will go berserk.
Stuff like Glalie's jaw being broken was also stated on the official ORAS website.
Sun/Moon is the first time it's been mentioned in a main-series game, but Mega Evolution having downsides and making Pokemon go berserk isn't a new concept.
