Red leaves on Rododendron

Hi all

I am hoping there is an expert amongst you on Rododendron.  I planted 2 beautifully big plants in the Spring.  They flowered and have new buds and have grown new leaves at the top but recently I have noticed that the leaves at the base of the plant have turned red - not all but some.  Some leaves, red and green, have fallen, but not many.  I also have azaleas they have some red leaves too but not dropping (I think this is normal as it needs to shed some leaves).

Is this normal for the Rodos?  If not what is wrong with the plants.  Is it too much rain after too much sun ( we did have a hot June and wet July).

what should I do to try to keep the plants alive?

thank you for your help.

cat