Managed to sabotage two mobos in two weeks. How come?

Alright so I managed to sabotage second mobo in two weeks. Both Biostar TB250-BTC PRO. I have not yet figured out exactly why but I'm thinkng its some kind of power failure. Reason Im writing here is to get some insight from you guys and also to warn people not to do the same misstake.

It all happend in the same rigg of 12 Sapphire Nitro+ RX 570. Im running two server PSU's @ 1000w to power all cards + risers and a normal 350 atx PSU to power mobo + cpu. However, I did NOT power the mobo auxiliary power through the molex cables. Reason why I did not was because I've read on multiple forums that it's not neccesary with selfpowered usb risers, and I've been running a 8x 280x rigg for a while on the same mobo without the auxiliary power. However I'm starting to think that this was the reason it broke because it couldn't handle 12 cards without it.

Everything was running under it's maximum capacity (whole rigg does like 1600w @ cryptonight atm). Suddently rigg just went offline and when I went to put it back on I couldn't get any screen and keyboard wouldn't get power. I've tried to run only the motherboard + cpu with the dvi straight into the mobo, but still nothing. Also tried hardresetting CMOS by taking the battery out for 10+ minues.

Note that this exact same thing happend twice now. Why do you guys think it happend? Because I didn't power the mobo through the additional molex connectors or could it have been because of something else? Cuz of a faulty server psu, breakout board etc? All insights are greatly appreciated. Also, do you think I can have a shot at returning them?