Hello, David. It could be lack of water and, in some cases, it may be too much sun/wind causing them to dry out. I have a MEM whose white blooms are suffering because it lost some of the shade protection. I am not sure how much water you are giving them (20 minutes using drip) as the amount of water is a function of the water pressure when you have drip but as an example... my drip irrigation runs for one hour to provide 1 gallon of water to the hydrangeas each time I water.
When the shrub does not get enough water, it aborts flower buds first (if there are any) and then flowers and finally the leaves. Blooms brown out probably as you are seeing. Leaves brown out from the edges inwards.
Just make sure we are not talking about the natural change of color that hydrangea blooms go through as their blooms mature. Some blooms turn a sandy/pale yellow/light brown color in June-July-August.