They crack in a number of places, depending on level of abuse.
I have a few where the center bottom is cracked. Fairly typical. The cause, I suspect, is that it's quite near where a back seat passenger would kick the area with their shoes on ingress/egress. It's really tight back there. Whether Ford could have foreseen this rough treatment, I don't know.
One other cause appears to be the old "Oh, you have a van?" joke. If you need anything in the back of your van, hit the brakes hard and any needed item will come forward. Any item in the fold down area of a fastback will surely find itself smashing into the seat back.
Another cause appears to be people who don't know how to remove these. The clips at the top are in a thin area and if you pull on the chrome straight back instead of lifting straight up the clips break and surrounding area cracks.
Defect or 50 years of abuse on a large plastic part?
The cost to restore these isn't that high but shipping these bulky things can hurt.