It maybe possible in your rig, that if you normally underclock/undervolt/underpower and this is a separate step, forgetting this step might be an issue. and by forgetting this step runs all 12 cards full out. Especially if the PS doesn't trip fast enough.
Thanks 64. This happened as soon as I powered the rig on. immediately a large POP and smoke. I think the pop came from one of the cards. maybe more than one.....not sure.