My PSU horror story

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.

Wouldn't the computer need to stay on for more than one millisecond in order for it to give you beeps? mine wouldn't stay on for that long at all when i first press the power button. When i try to press the power button again, nothing would happen at all this time until i unplug then replug the psu power cord then the cycle repeats.

damage claim form with corsair (the company i got my psu from) has already been filled out and i sent in my computers to them for inspection. If they find out that the boards were at fault, i'll then send everything to MSI. I also checked the older motherboard for bad capacitors very closely and they were all in perfect condition.

PS. the i7 and the 680s were never overclocked. I never even got to use my i7 since the new motherboard wouldn't post. I also never overclocked anything in my life so lets go ahead and just cross over that possibility.