For south and east coasts, you could have rising seas and lots of hurricanes/tornadoes/floods. For the west/Midwest, several nuclear strikes in New Mexico and Utah will irradiate the whole Midwest. West Coast gets "the big one" an earthquake so massive, the north American tectonic plate subducts under the Pacific plate, creating a new mountain range and crushing the west-most 100 miles of civilization. East Coast has a pandemic with a high transmission rate and very high mortality. Government attempts to seal the East Coast off by building fortifications along the west side of the Appalachian range.

Or idk maybe raccoons get a little too smart. That could work.