As with all evergreen shrubs, each year older foliage (usually leaves 3 yrs old or more) will turn color and drop - This is normal and not a cause for concern if it's the inner and lowest leaves.

Premature leaf drop can occur following a period of drought and is usually preceded by drooping and rolling of the leaves. Again, oldest leaves are the first to drop. Higher leaves may show browning at the leaf tip or edge. Leaf drop can also occur following extended soggy periods ...

There are a lot of colorful leaves either on rhododendrons (lower and inner) or on the ground under them here right now, adding to garden chores because they decompose so slowly, they really require picking up. Do you have reason to think you are seeing more than normal leaf drop, i.e. is this years or last years growth being affected?