Some speculating.
If the field excitation output from the controller got wiped out, the controller might see it as an open field and give 5-red flashes.
If the armature excitation part of the controller is still intact, it would be feeding a stalled motor, so the armature current would be whatever the controller's amp limit is and that could warm up all the high current cables. The B+ cable getting hotter than the rest means it has more resistance.
With high amps flowing through the armature, some current is going to be induced into the field windings because the controller's output is PWM rather than straight-line DC. That might create enough magnetic flux get the armature to twitch a bit, which might make the cart shudder.