Via Mike Munger, a supercomputer figured out why a two-seam fastball drops suddenly:

As the ball spins, its stitches interact with the air as well and creates what Professor Aoki calls a “negative Magnus effect” that causes an additional downward pull. As you might suspect, the influence of the ball’s seams is small, and in order for a significant negative Magnus effect to be created Professor Aoki says that the seam must be in contact with the air flow for about a third of the rotation.

SoraNews24.com

There are a couple of GIFs at the link that show the difference in seam interaction between and four-seam and two-seam fastball. Very cool.