This is most probably to unclean bottom. When a boat is kept in water staying in the same place, one side might catch more organizims than the other. The main reason is the sun. The sun effects growth. If there is something wrong with the
propellor, shaft or the rudder the rudder will not be loaded on one side but the boat will travel sideways without any load on the rudder.
If the boat is in the water now it means the bottom is cleaned. Recheck it.