After adding one, I found out that when the display goes to sleep, Windows sees it as a disconnection, and 'helpfully' wakes up to reorganize all the windows onto one display. That wakes up the Display port monitor, which triggers another reorganization, and the cycle begins again.

I ran into this after building a system and it was very frustrating. IIRC my problems were related to my network card and having virtualbox installed. Also turning off wake for my mouse helped (bumping the desk would turn on the computer).

Wasn't any device, like network adapter and power diagnostics returned nothing.

Solution: reset motherboard NVRAM. No more mystery wakeups!