Endeavor triggered Volt Absorb

I'm working on completing my Ultra Sun Pokedex and was grinding XP in the Poni Fields.

My Zeraora was up against a Cottonee. It was on the left side of the battle after I KOed the first Cottonee after it called SOS. The Cottonee had set up Sunny Day before I KOed it.

The Cottonee used Endeavor and it somehow triggered Volt Absorb which healed Zeraora.

How the heck did this happen?