You were right to kick the mech.They shouldn't be spamming knockbacks on targets.'You can target other enemies' is not a valid excuse.Thats encouraging bad play.You should be focusing on a single target to make tanks job easier.By encouraging the idea of attacking multiple targets at once when it's not aoe skill based you are making the tank do more work to hold agro on more than one mob at a time.
By the sound of it though the Dragoon wasn't helping the situation though.More than likely by not asking the mech nicely and giving out to him instead probably encouraged the mech to knockback on purpose to annoy the Dragoon.