If everything else seems to be in order, I would say nutrients or fumes from a building project or furnace?
Anywho, salts in many fertilizers can be detrimental and if added to city water that already has high dissolved solids/ salts it can be bad.
The same hormone is sent into the plant when salt is too high as when the plant is going through drought. If you are sure there are no fumes or argon or anything crazy like that. They could be canaries warning you of a danger you cannot see.
just read last post. Glad you got it resolved.