Well, it happens.

I mean, if the 750Ti is fully seated in the PCIe slot (you said you tried them all) and the 6-pin power cable is plugged in, then there's no reason the GPU shouldn't work (especially the fan on it); and since you can plug the old GPU back in and it still works would only indicate to me that the 750 Ti is just plain shot.

If you get another one and it blows up too, then it could be the PSU. /shrug