An American Airlines 737 (AAL2587) was on final to land at KAUS when a Cessna 182 apparently turned unexpectedly into the path of the 737. The controller spotted this and attempted to vector AAL2587 out of the way. At the same time the AAL2587 flight crew received an RA from their TCAS system. The 737 aborted their approach and landed safely a short time later without further incident. The NTSB is investigating.