Well there are a lot of reports of PCI slots crapping out, I came home to this in my rig that had 3 RX-480s in it. They all went back for a refund today.
Can you answer the following?
Were you dual mining?
Overclocking?
Increase Power Limit?
That is what happens when you draw more than ~ 300w thru the 24pin atx connection and the reason we use powered risers. Which in the case of the rx 480 does not seem wise as the meager 6 pin pci-e connector isn't upto the job, particularly if you overclock and or increase power limit.