This sentence says it all I believe. I've been keeping sps for several years and have never only needed weekly glass cleanings. While at Reefcurrents last weekend a WWC employee(I can't recall his name) was lecturing about the store, specifically their main sps tank. Someone from the audience asked what was the nitrate level in the sps display and he went on to say every system is different...then he paused and said "if you are not having to clean your glass every day or every other day you are starving your sps. SPS need nutrients to thrive".
Sounds to me it was a drastic change is what did it. If it was gradual perhaps they could have coped with it better. My opinion is to continue to feed regularly, limit water changes, and run skimmer sparingly.
Good Luck. I'm sure your sps will bounce back.