Crawling in induction motor is caused by?

Crawling is caused by harmonics effects on the motors rotating magnetic field. If harmonic levels are high enough they can become the dominating factor in the rotating magnetic field causing the motor to run at a much lower speed than designed.

http://www2.sea.siemens.com/NR/rdonlyres/4058E086-2AFD-4C47-B9B1-0FCF4428FA39/0/MAGNETICNOISEININDUCTIONMOTORS.pdf