Why would pH drop suddenly in a saltwater aquarium?

Salinity can affect ph. You don't mention your specific gravity. Some say it doesn't affect ph but I've had fish only tanks and true reef tanks for over 10 yrs and ph can go up or down if you don't maintain the same level. I keep all three of my tanks at exactly 1.023 and have never had ph issues UNLESS I don't add water lost due to evaporation.