There are two reasons why DEN is such an outlier in O&D traffic.

First, Denver is the most isolated major city in the US. Nowhere of any significance is within driving distance; this causes higher than normal amounts of air traffic (MSP and SLC also have this issue).

Second, the F9-WN-UA battle for Denver has caused DEN to have significantly below average domestic airfares. This has caused demand for seats to skyrocket over the last few years (much to the detriment smaller airports in the region).