Alfred Wegener thought that continents drifted because of
centrifugal forces; today the picture is more complicated,
involving convection in the upper mantle, the motions of the
seafloors spreading from the mid-ocean ridges and their densities
varying with age, gravitational forces, tidal forces, and
centrifugal forces. Scientists have differing theories.
See also in The book of science:
Readings on wikipedia: