If it is mites, check the water once the snake gets out, there will be some tiny black spots in the water, these are the mites.
To see them easier, tip the water into a bucket of water filter by paper towel, this should show them if they have mites.
To get rid of them, get your self some Top of Decent and follow the instructions. Repeat the process after 5-7 days.
Also remove all substrate and put fresh stuff in once the enclosure has been treated. Repeat the process after 5-7 days
Hope that helps.
Edit:
Just re read your post.
They seem to do it because they like it. Some do some dont, I guess like some people like to swim and some people don't.