I have 8x280x on same rig and everything is fine. I think the problem are new graphics cards (Rx series), with old card (HD 7950 or R9 280X) my rig start normal. 8x 280x works perfectly on same windows 10, and same 16.9.2 driver.
What is your motherboard, processor and operating system where 8 of the GPU?
What to do with the motherboard BIOS?