As the water is pulled down into the opening, the water particles fight for the smaller space and push each other to the side. This pushing and nudging by itself would not necessarily cause the water to spin, especially in a perfectly-shaped funnel and no other directional influence on the water. But a perfectly-shaped funnel never exists in nature. There are inconsistently-shaped rocks or other obstructions that force the water away from them. This initiates a spinning motion that accelerates as the water is pulled by gravity.