It's the stock controller that comes with the machine. They are well know for poor reliability in the smaller '20' mills. I have no reason to assume the larger controller is any better, if indeed it is a different controller.
They have a load balancing circuit that I think increases the voltage to the motor under load, thus increasing current and raising torque when you are taking a heavy cut. I haven't adjusted mine, but when the load on the spindle goes up my motor actually starts to increase RPM slightly, so something isn't perfectly adjusted. But it has always been like that. I'll open up the controller and see if there are any adjustments I can make to decrease the arcing.