I've got a friend-of-a-friend who's a guide in Rockport and he jokes about how clients (non-fisher types) always ask him why mullet jump. He said he came up with lots of "reasons", but that his best one was "to find out where they are".

The real reason, as told to me by the intermediate friend, is that mullet have parasites that grow between their scales, and they are jumping to slap themselves on the water's surface. Guess they gotta scratch the itch! (this person was studying aquatic biology) Maybe it's BS, but it's the best explanation I've heard so far.