New
#11
My thought is that the first board died and the second was DOA (as borne out by some Egg reviews saying "No post" and "would run for a few seconds and stop". If you're capable, I would check the first board for bulged capacitors or tiny burn spots indicating a short. Maybe you have a friend who is keen on spotting motherboard problems if it's out of your league (certainly out of mine).
The answers to your how, why, when questions are will probably remain a mystery. However, looking at your hardware I would assume you're overclocking at least the CPU (K version) if not the GPU as well. If you are, all bets are off regardless of what others may say about "overclocking does no harm, etc." It could be true, it may not be and probably isn't.
The board has a 3 year warranty, through MSI, so I would contact them to RMA the board(s) subtlly marking them to determine if you get the same board(s) back after the RMA.