Be careful with your setup, I dont think 1200W PSU could handle 7x1070. I havent used that mobo yet, but basically your rig could boot with 6 GPUs, it means it also can boot with 7 GPUs as well.
I agree with you. Let's say this is under standard conditions (no OC/Undervolt), 1070 with 150Watt TDP, it means OP needs more power space when OP start mining process. And maybe the power issue is what caused your system didn't want to boot.