It is difficult to achieve such perfectly balanced and still conditions that water will drain straight down an outlet at the bottom of a tub, sink, or toilet. There is almost always some residual motion in the water that starts it spilling into the drain faster over one side of the lip than the other. After that starts, it simply becomes more and more noticeable. Often there is an irregularity to the drain lip, or to the shape of the vessel that's being drained, which sets up the imbalance.
Most people believe that the swirl will normally go in one direction in the northern hemisphere, and the opposite direction in the southern hemisphere, and that the swirl itself is a result of the rotation of Earth. They refer to this as the "Coriolis effect". Even though this is very widely believed, it is not true.