It's just the simple fact that DEN is a UA fortress hub. They control about 70-80% (can't remember the correct percentage) of the flights here. If a competitor introduces a very low fare in/out of DEN, UA strikes back by lowering the fare of a comparable route to/from the competitor's hub. From my observations, it's very rare for UA to match competitor fare sales in/out of DEN (with the more common exception being F9).