Southwest Airlines Boeing 737-7H4 takes off from Hollywood Burbank Airport on September 16, 2020 in Burbank, California.AaronP/Bauer-Griffin | GC Images | Getty ImagesSouthwest Airlines is pushing into United Airlines’ territory.Dallas-based Southwest on Monday said it plans to add service at Houston’s George Bush Intercontinental Airport and Chicago O’Hare next year, two United hubs and the cities’ larger airports. It marks a strategy shift for the low-cost carrier that has relied mainly on secondary airports in some of the largest U.S. cities. Its operations in those cities are still centered on Houston’s Hobby Airport and Chicago’s Midway International Airport.The shift comes as airlines rethink their networks after the coronavirus pandemic devastated demand. Business travel has largely been absent, forcing carriers to focus on vacationers with beach and other outdoor destinations.Southwest isn’t the only carrier to sense opportunity amid the lull in traffic to once-congested airports. Chicago-based United is planning to return to New York’s John F. Kennedy International Airport after a more than five-year absence in early 2021, CNBC reported last month.