Midway is an airport sitting right in the middle of a Chicago neighborhood. The runways are very close to 55th and Central.
Southwest is fighting now to repeal the Wright amendment, which would allow Southwest to fly long-haul flights out of Love Field in Dallas. Love, like Midway, is a small airport with little buffer between it and city neighborhoods and streets.
If Southwest is allowed to expand its traffic out of Love, with other airlines thus doing the same because Southwest refuses to fly out of the larger airport, it's just a matter of time till what we see tonight in Chicago happens in Dallas.