Well, Kuja did have something prepared, but since Bowser's gone to so much trouble to immobilize himself and render himself blind, how could Kuja possibly do anything other than take advantage?

He keeps the spell primed, and books it for where Madhuri's having a traffic collision. Shame, really, to abandon a perfectly good line like that- but he makes sure to smirk for the cameras before he leaves. If he was trying to get Bowser to flinch and go on the defensive, then that's alright. He's not retreating for a tactical advantage at all, he's bluffing. It's all part of the plan, and he most certainly is not improvising in the heat of the moment. That would be unsporting.