Some post mint process has pounded on the rim to make it flat and start to expand.
This is sometimes called "Spooning" because it can be done with a spoon. Similar effects occur in arcade machines where the same coin is used over and over cycling between the change machine and the game machines until someday it escapes in somebody's pocket.
Also this effect can be seen in commercial clothes dryers - called a " dryer coin"
Search for those terms here and you'll find plenty of examples and images.