With wiring like what I see, I would not be surprised that something like this can happen, You have not used Shielded cable and most likely don't have a Ground from the spindle to the VFD Drive
12,000 is also most like to slow for your air cooled spindle with only 2A it would not have much torque and would over heat very quickly it the cutter was loaded up
Things to know more about wiring and drive Parameters
Over load at low speed most likely caused your problem helped by bad wiring installation