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I'm not joking. I've since swapped to display port after reproducing this strange behavior multiple times.

Looking to see if anyone has an explanation as to what is happening.

Powering on my monitor, the PB278Q when plugged into my Sapphire R9 290 Tri-X OC causes the fan LEDs, the LED backlit logo on my H80i to light up and then turn off within 2 seconds.
This is pretty disturbing behavior... and it's only with the discrete graphics card. Did it over onboard HDMI... nothing. Tried with my HTPC's Gigabyte GA-F2A88XN-WIFI and couldn't reproduce it either. I did NOT put my 290 in my HTPC though, not worth tearing both rigs apart.

Oh, and it does it with my power supply unplugged too.

How *()(#)$! baffled are you now? There must be significant voltage travelling from the monitor through the HDMI 1.4a cable to the 290's HDMI port through the PCI-E slot then through the 6 and 8 pin on the GPU and possibly mobo to the 8pin and 24pin connectors to the PSU and back out to the sata/molex power cables. Seriously. I can't even think on how.

To reiterate, I am no longer using HDMI because of this and using display port which does NOT cause this issue.