Did you acclimate your SPS frags prior to putting in your tank? Did you introduce the water from your new frags into your tank?
My theory is something in the water from your newly bought SPS came into your system and pissed everything else off. The newly bought SPS may have acclimated to the conditions in their previous system but once that water was introduced into yours, that brought about the demise of your current stock.
I try to dip, then acclimate all corals to my water by slowly mixing my water into the bag. When it is time to put the coral in, I aim to remove the coral entirely from the bag and discard the bag with the water still in it, only putting the coral itself in my tank.
Sorry about your loss!