The roughness of the seam at the front of the ball (on the left as shown
here) makes the boundary layer cling much further around this side. On the
smooth side the layer breaks away early on. This produces an asymmetrical
pressure distribution over the surface, and the resulting sideways force
component makes the ball swerve.
The seam stays in position because of the backward spin given to it as
bowled; the direction of this is indicated by small arrows along the central
line of the seam.
(The drawing is not to scale : the thickness of the boundary layer is
exaggerated in order to show it.)
Ball moving below the critical speed, but flow rendered unsymmetrical by
the effect of the seam. Ball viewed from directly above.