From my perspective, it appears as if they've been raising the fares through IAH.
Anytime that I've tried booking PHX to DFW or AUS, the IAH connections, which I've been doing for years, area all at the bottom and priced $200-300 higher than other options. Most run through DEN but I also get LAX and SFO connections at at cheaper fare (and an RJ from the west coast to DFW ).

Happening once or twice, I assumed it was the luck of the draw and that the lowest fare buckets had already been sold on the IAH flights. But it seems to be consistant.
I wonder if maybe they're trying to force some traffic thru less congested hubs and/or leave IAH open for more long haul traffic ?
Or is it as simple as CO pricing itself higher than UA ?