The lil plastic clips and the piece that the bumper connects to that wear out and cause this. those two no longer sit right in the metal bracket that is atached to the fender. The front dosnt seem to sag to much, from my experience.

Bigger problem on them is the corner mounts breaking when the bumper is removed, or just from the owners abusing them somehow.
all the bumpers I have seen have had the bumpers foam painted over with a sealant.

my 2.3 cents....

TF