Electrical gremlins under high Amp draws
Lamp ignition on a HID lamp is electrically very noisy, being as it involves very high voltage pulses for arc ignition. 

I would expect the problem here to be RF pickup by the USB leads during lamp ignition (You may also see HDMI dropping out), cure is probably some ferrites as common mode chokes on the ballast DC connections (And maybe also on the USB leads). 

I see something similar with the workshop PC when welding for much the same reason.