If you look on the inside of that plastic piece (you can just pull it back) there is a metal tab on the seat that the plastic is suposed to slide over.
When people exit the car by sliding their butt over the edge of the bolster it puts pressure from the cushion onto the plastic and the plastic brakes.
I have seen many 4runners with this problem including my own.
You need to either glue the plastic back together, or zip tie the two pieces as
@
kball
suggested.