Because your alkalinity changed suddenly, I think that's the point he was trying to make. You keep saying you measure everything, but we are assuming you weren't measuring alkalinity daily while you were gone, hence there's a chance something could have happened then which you don't know about.
It could have been the water change (what salinity did they mix to? How did they test it? If the salinity was off, the alkalinity and calcium and other parameters would have been off by the same amount. Maybe they corrected the mistake or it self-corrected by the time you returned, thanks to your dosing pumps). Or your dosing pump may have hiccuped or been disturbed while you were away.
Or it could be something totally different. You said the lighting didn't change, but the light intensity at the actual surface of the coral depends on many factors besides the light unit itself. Maybe the LFS ran carbon or a filter sock during or after the water change? Things like that can cause a sudden jump in lighting intensity: suddenly running lots of carbon, or even just a large water change, can remove enough organic matter from the water column to significantly increase clarity.
Maybe your skimmer was not maintained similarly to how you normally do it when you're not on vacation?
Did you get a chance to check for pests?
It's really hard to diagnose because none of us (apparently including you) were there when this happened. Acropora can be very sensitive to very small changes, and the damage can take days or weeks to show. I agree with karimwassef, your best course of action is probably to cut away the empty skeleton and hope for the best, assuming there are no visible pests.
