Per Southwest, Flight 31 exited a taxiway (not runway) after landing, on the way to the gate.
Statement below:
At approximately 5:20pm local time, Southwest Airlines flight 31 from Houston Hobby Airport to Nashville International Airport exited the taxiway shortly after arriving into Nashville, as the airplane was approaching the arrival gate. The 133 passengers and five Crewmembers were safely evacuated from the plane and bussed into the airport, our Employees are currently working to support their needs. We are coordinating with local airport officials and responding agencies. The Safety of our Customers and Employees remains our primary focus.