High alkalinity can also be to blame (goes along with pH shifts). Whats your alk at? Also, freshly mixed saltwater despite being "instant" is not exactly that and I have found that adding any appreciable amount of fresh saltwater is very stressful for corals and other inverts. Could certainly kill turbos and bleach corals anyway. IMO saltwater should be mixed and allowed to sit for at least 6 if not 24 hours before using it. The longer the better basically and if you can bring it up to temp for large water changes as well, thats ideal.
I wouldnt say I have seen a conclusive connection but certainly corals can be stressed (which IME can lead to bleaching) when bulb's shift/reduce output. I wouldnt rule it out anyway but the easy question if you have changed nothing else is how old are your bulbs?