How is this possible

Had a match against mega starmie and i was using a Beedrill deck i put together. And after getting terrible luck as usual , my opp was taking out my bench an active pokemon. Finally got something to work with an attack. My next turn i attacked to take out starmie… but instead it didnt an moved to the bench an just evolved his mega abomisnow into the active an put energy onto him an attacked me!! How!?!? He didnt play any card that i saw to make that happen. Plus ive never seen it happen before! Would pretty much be a top deck to use imo. Is there something i dont know about these 2 pairing together?