There's a good chance it has to do with one of the following: Mites, dehydration, stress, etc.
I would try the following things: Put hiding spots (good ones, check the faq on proexotics.com for examples) on both sides of the enclosure (so that the snake can choose which suits its temperature needs. If that doesn't fix the problem, then trying adding moisture to one or both of the hiding spots (creating an area of higher humidity). If that doesn't work, dump the water from the dish into a clear, container, and even rinse the dish (into the clear container). Set it on something white (paper) and check very carefully for any small specks that could be mites. If the snake has mites and is soaking, there are going to be some dead ones at the bottom of the water dish. I discovered this trick accidentally when my ball python had a mite infestation, and it is one of many good ways to check for mite infestation.
In captivity we tend to forget that these large snakes spend all day hiding in the wild. Why not let them hide here too? Most seem to do well without any hiding spots, but of course there are exceptions, and it is well known that snakes will trying to hide inside anything that can offer some sort of security, even if it means being wet the whole time. Better to be wet than eaten, right?
So maybe try some hiding spots, to calm the snake down if it is in fact stress, and you could even put a designated hiding spot (or two) that have higher humidity. Maybe an opaque rubbermaid container with moss. Offering more choices is something I'm doing with all of my reptiles now, and it seems to work very well.
Good luck!