Good Morning,
It could be that the heavy load of flowers are causing some older leaves to drop off. It could be the bush may be somewhat stressed and if the soil pH is high, that could affect leaves. It could be a lack of nutrients in the soil and the plant cannot support everything. It could be lack of consistent moisture or moisture going deep enough.
Hydrangeas For Mississippi Gardens | Mississippi State University Extension Service (msstate.edu)

What's wrong with my plant? : Garden : University of Minnesota Extension (umn.edu)

Here are references for leaf spot diseases to watch, which can become a problem especially since we have had a wet several weeks-http://www.tnstate.edu/extension/documents/Foliar%20diseases%20of%20Hydrangea%20FBG%20022916%20G1.pdf
https://hgic.clemson.edu/factsheet/hydrangea/

If yellowing continues to spread and your drainage is good, I would get a soil test so you know the soil nutrient and pH values.
MSU soil test self mailer can be purchased at your extension office or,online here-https://www.canr.msu.edu/resources/soil_test_kit_self-mailer


I hope this helps.  Thanks for using our service.