Negative phase sequence components create a rotating magnetic field in the stator which moves in the opposite direction. This causes a decrease in the torque developed by the motor. The motor will thus have to draw a higher current for the same mechanical load.
The rotating magnetic field which rotates in the opposite direction induces voltages in the rotor. These voltages have a frequency that is double the system frequency. Since the frequency of this rotor voltage is higher, it flows on the surface of the rotor due to the skin effect and causes surface heating which can lead to motor damage.