Gravity. It's the law.

scienceisbeauty:

A rapidly spinning, dense fluid has a lower density (orange) fluid poured onto it, at the center. This forms a buoyant lens whose lower interface is sloping relative to equipotential surfaces (the equipotentials are identical paraboloids, like the free surface when the fluid is totally spun up). {Note that the free surface of the fluid is just above the colored region. This is a strong optical lens, making secondary images above.} This symmetric eddy will continue to spin, relative to the underlying fluid, for many hours, until its available potential energy is used up.

 Source: GEOSTROPHIC ADJUSTMENT AND GEOSTROPHIC FLOW, UW School of Oceanography

Not only is this absolute eye candy, but it has a great physics explanation behind it