um the same thing that causes any circuit breaker to trip your drawing too much load though it and it shutting down that part to keep your house from burning down

you have a faulty power supply, why? I dunno let me look into my crystal ball, oh cause its broken

power supplies are somewhat tricky dangerous things to work with, I wont even attempt to repair one (and I helped design the ones we use in our products at work) its often so much easier wiser and safer to just replace the unit