There's no way to answer your question without much more information. You said there is lots of leaf drop every year. This is normal for evergreen plants - they drop their older leaves at some point, often around the time they grow the newest ones. So I am not sure why this year is different and you are concerned about the survival of the shrubs. You're correct that there has not been that much change in weather/climate - Photinia is very tough and can survive a lot that would stress other shrubs. Prolonged drought, or drought damage over several years would be the most likely to damage it.
Photinia is also, in our climate, very prone to a fungal disease that causes spotting on the leaves. If it's bad, this can cause leaf drop. Have you seen any sign of that?
My suggestion at this point is simply to keep an eye on the plants, see if they grow new leaves, and if they look normal after that. If they seem stressed - few leaves, thin growth, fungus, drooping, anything else out of the ordinary - then describe it thoroughly and send pictures.