I presume you fogged the paper by not handling it in a suitable safelight (red, or orange, etc as specified for that type of paper). It is unlikely that you overexposed the paper so grossly at one second that you pushed it straight to black.
Did you handle the paper in safelight when you opened the packet and put the paper under the enlarger, and until it had been in the fixer (after the developer) for about 10 seconds? If you handled it in white light at any stage before it was fixed then you fogged it. If you opened the packet in white light you fogged every sheet in that packet at least at the end you opened.
There, I think all possibilities are covered, except, of course, the possibility that someone else has already fogged the paper before you got hold of it. Was the packet sealed when you got hold of it?