They're really called "lenticular" clouds, and they're caused is formed when warm, moist air runs into the surface of Mt. Rainier. The mountain's topography forces the air upward, which cools and condenses the air -- turning it into a cloud.

As the air sinks back on the other side of the mountain, it dries out and the cloud dissipates. That's why it just hangs over the summit area.

(Although it looks like it is "hanging" over the mountain, air is continually flowing over the summit.)

For more information, check out the KOMO Weather FAQ.