I could be wrong here, but just wondering out loud...I know in cases of IRROPS most airlines are willing to send you to another city within a certain distance band...I wanna say it's 500 miles but I could be wrong. So, if I HAD TO GUESS, I'd say Copa offers to get you to IAH and calls it a day, saying that's far enough, because IAH-AUS is 140 miles according to GCmap. I wonder if he had been ticketed to Seattle for instance, if they would've said it's IAH and that's it.
Just a hunch.