Could be a number of reasons.

Sometimes BP's soak after a big meal, which was 2 days ago, I doubt this is the case.

Sometimes BP's soak if they are in shed, which probably isn't the case.

Most likely Mites if he/she has been soaking nonstop for days. Mites are little black specs. Check your water dish very carefully.. they look like black dust particles floating in the water.

Could be because he/she feels more secure in his/her water bowl then anywhere else. Make sure you have at least 2 tight fitting dark hides for him/her.

Check your temperatures and make sure the hot side is not too warm.