From the video I would guess that your power supply is over current tripping (which a 750w should notdo if everything is correct.
Chassis mouting screws are the first thing that come to mind as not all mother boards have the same mounting holes. A stud on the chassis short circuitng the board could cause the PS to trip. (could also damage the mother board)
The removal test tends to lead this way also.
I would check the PS 5v stand by output rating to see that it is enought.
You could also try removing everything from the board one piece / connector at a time. any USB connectors or devices, then RMM3(if installed),Front panal switches, memory, HDD's , CPU and anything else installed and then connect the power and AC.
With nothing installed , the standby voltage LED should be on solid and the status LED should blink amber for about 30 seconds then go green indication the BMC is up.
Yours is loosing 5v from the power suppy , over & over & over.....