I'm not the expert on motors of any kind but in all types there are factors that limit the maximum torque the motor can deliver and others that limit the power. Since power is equal to torque times speed there will be a range of speeds where the torque is at it's maximum and the power produced rises with speed. Once you hit the maximum power possible from the motor and/or driver the torque must fall as the speed rises. Eventually all the power is used just to turn the motor itself and you've maxed out.

Kit