The force isn't slight. If you've done any straight-wall cartridge loading with conventional dies, you will have noticed you can get a surface bulge mirroring the base location of the bullet that is only clearly apparent on one side of the case. When that happens, the uneven bulge corresponds to tilt in the bullet that pushed the side of the case in one direction and pushed the mouth in the opposite direction. That requires lateral force. Where it comes from is the tilted bullet acts as a one-sided wedge being driven into the case mouth. It bends brass easily for the same reason you can easily move heavy objects apart by driving a wedge between them. In effect, it gives you a lever arm that magnifies the downward force in proportion to how many times the wedge is taller than it is wide, and it applies that force perpendicular to the direction the wedge is driven into.
I used to get a lot of those uneven bullet base bulges loading 45 Auto on my Dillon Square Deal. I finally made a powder tube for its measure with a Lyman M profile expander so I could set bullets straight up and down in the cases for seating and crimping. That put an end to the uneven bulges. The M-die is a way people make standard seating dies seat bullets straight. Lyman even boasts about this on their site. It works very well.
I used to get a lot of those uneven bullet base bulges loading 45 Auto on my Dillon Square Deal. I finally made a powder tube for its measure with a Lyman M profile expander so I could set bullets straight up and down in the cases for seating and crimping. That put an end to the uneven bulges. The M-die is a way people make standard seating dies seat bullets straight. Lyman even boasts about this on their site. It works very well.