This could happen because the Bullet Seating or Powder Through Expanding die is adjusted too far down into the press.
This places as much load as you can manage to apply to the press lever times the mechanical advantage built into the lever linkage along the back edge of the shell plate. The shell plate carrier is not well supported along the back edge--it is only supported under the sizing die in front. This puts tremendous strain on the shell plate carrier casting in a way it wasn't designed to withstand.
