Short circuit on cable
or
short circuit around switch area internal
more likely pcb fault in ball assembly or the main motor itself.
Try turning off head motor next to main switch, then power up. If it doesn't trip then it's the head drive pcb - if it still trips then it's probably main motor, If motor bearings seize then that can cause this.
If you are not confident in tracing this type of fault then it needs to go to a competent repairer.