You checked in at AUS for both your AUS-IAH and IAH-LAS. When you checked in you were put on the standby list - maybe even OLCI at co.com - and was #3 in AUS.

It is possible that three other PLATs on higher fares or any elites on Y checked in for their IAH-LAS - originating in IAH - after you checked in at AUS. If one does not clear via EUA or Y fare upgrade - then you go on standby list at the time you check in - and it gets reordered as folks check in.

With all the new Trump style condos and high end hotels - F sells to LAS.