Contrary to what most believe BP rarely soak and usually when they do there is reason and the only thing you can do is go by elimination.
They can soak for the following reasons
Mites, this is often the main reason.
Hides not providing enough security and using their water dish as hides.
To hot, the temperatures are not measure properly or measured where they should be and the animal is trying to escape the heat.
Too large of a meal, the animal soak to relief the pressure.
Some animal will soak when in shed but again it unusual.