I always give my snakes a bowl large enough to soak their whole bodies comfortably. Sometimes there is no rhyme or reason to the soaking, but it may mean your humidity isn't high enough or that your temperature is too high. I would just do a quick check to ensure everything is at a comfortable level for him. It could also be due to mites if the snake is soaking often, or it could be he is about to shed.
i have one snake who loves water and soggy hides despite having good humidity levels, and one snake who hates water touching her body like it would cause her to melt.
but that is all of my knowledge on it.