Why so much rust at the bottom of the rear door?

[Pete99]

I just went from my beloved '97 2D 3.0 4WD with 209k and almost no rust to a 2002 4D that's more typical for the Northeast. While shopping, I learned where many common rust areas are, one of which is at the bottom of the rear doors on 4-door models.

What's the process whereby this spot so typically gets rusted? What's the water pathway and why does it stay on this area enough to rot it?

I would love to get ahead of this one.

 

[85_Ranger4x4]

The pinch seam at the bottom collects stuff. There is a tsb out to unpinch that seam, sandblast it, repinch it, seal with an epoxy and repaint. At least there is on 97-03 F-150's but Rangers do the same thing. The front doors and tailgates do it too.