Based on the fact it was a "Sweetwater" area congregation (southwest of the airport near) I see no reason why they were anywhere near the entrance to the Airport. Sweetwater is right next to the Turnpike
The only thing I can think of, (and I base this on my own recent experience) , is that the driver encountered one of the many detours and wasn't able to find an easy enough place for a bus to exit the freeway to turn around once he went wrong. One detour led to another and he wound up only being able to take a route through the airport. I say this because this is exactly what happened to me. I was in a car so I didn't have a problem with an overpass but had I been in a bus it would have been much different. Either way you'd have thought a bus driver from the area would have planned his route better or known that he was too tall to pass under the concrete abutment. It must have been quite a wollop !
The other possibility was they were picking up someone from out of town who was flying in for the assembly. ??