Its very strange that it would freeze because it was pulling too much power. I think on stock voltage those GPUs pull maybe ~150-160Watts each, so 6 of them is around 1000kw. The PSU should still be able to handle it because if I recall the G2 1300W PSU was single rail. So it didn't matter which way it was delivering power.

If you got a quality PSU you can run it at 80% load most of the time and not get any issues. I remember back in the Antminer S2 days, those pulled 1000Watts out of the box and the PSU included was a 1000Watt PSU. And basically a large percentage of them ended up failing since they ran at 100% load non-stop but there was also tons that ran fine at 100% load for years. Hence 80% load on a quality PSU shouldn't be an issue.