when breaking rock instant mega evolve

[zdenek]

i just got my tyranitar and i try mega evolve then igo to cave and start breaking rocks and the tyranitar was instant mega evolved

[Jonathan Peters]

I do the same thing, and does not happen for me. I have a delta gallade to smash rocks, and he has his mega stone, but I still need to activate the mega evolution if I want it to. might be a personal problem, or possibly just save and soft-reset