Hakkel realized that there weren't any traps in the room because the room itself was the trap. The fake room was there to lull him into a state of false anticipation. Hakkel didn't know if Micah had correctly guessed that he would smash down from above the ring or not, but doing so had put Hakkel in a bad position.
Swinging his axe down like that and smashing open the floor with more force than necessary had destabilized Hakkel and shifted his balance.
Naturally, as experienced a fighter and as good at controlling his body as he was, Hakkel could readjust and prepare to land on the floor beneath him. But when that floor disappeared below him when he touched it, his readjustment had the opposite effect, and he started falling in the air, dragged down by his axe.
