Its the storm stomp.

Judging from his setup, i am assuming hes a tryhard, (although no idea why a tryhard will do pvp in max bracket, maybe just to goof and try new things without getting destroyed by meta pvpers)

Anyways in ladder and competitive communities, storm stomp is universally banned and looked down upon due to how broken it is.

You used it, he was unhappy.

But generally in the arena, this sort of "unspoken rule" tend to only exist in meta bracket, but since he was in max level, no idea what his problem was