So just what exactly is the cause for this sort of change? I just can't see what sort of catastrophic change would cause this that we would actually be witness to in our lifetimes.

What do the coastlines of other continents look like after this happens?

Somehow I doubt that if the U.S. were to look like that in a short period of time, that there wouldn't be very far-reaching changes to coastlines in other areas of the world as well.