[OccamzRazor]

Grab a multimeter and start checking for current and continuity. Something somewhere is making a short and supplying current when it should not be from the sound of it. Check if it stops when you pull out the pwm cables from the speed controllers, if it does then check the cable, or speed controller and replace it to see if the problem repeats.

If that does not work start testing wires and systematically checking the wires from the distribution board to the speed controllers. Comparing amperage from those wires should tell you which component or wire is the problem.

Stay safe and be careful with that problem!