Sounds like some sort of acclimation issue, if as you say your water is "fine". If you have had healthy SPS in that tank for a while, it probably is okay...so either the new corals were unhealthy to begin with or there was something in the transfer into your tank that caused them to bleach.

If these are acroporas, take heart as acros sometimes just revolt when moved from tank to tank. Salinity, temperature, ph, Ca, Alk levels all have a big impact on the corals. It's also at least theoretically possible that there's a salt incompatability. A while back lots of people experienced bleaching switching from IO to Bio-assay salt. When I was using Bio-assay, any coral that I got from someone using IO was likely to initially bleach.