I suspect just the temporary heat - the higher than normal temperatures are causing the leaves to transpire water faster than the roots can take it up.

Are they perking back up in the evening and looking normal in the mornings? If not, let us know as it may be something else.

Just as it is normal for the leaves to curl in winter, it is just as normal to wilt in the heat of these very hot days. As Morz8 says, the problem is if they are still wilted the next morning. Then their either need water or have a problem. Do not water too much. Wet roots die of root rot in hot weather.

If leaves wilt on one side and do not recover in the morning, then that could be a drought problem.

Here is a link that might be useful: Rhododendron problems and solutions.