Chicago is a unique co-hub. 80% of the flights in and out of ORD are either United or American, with United having a share of slightly more than half of that 80%. I'd imagine that American would have a much larger share of traffic out of DFW as it's a fortress hub where a large majority of the flights are American. In fact, on that very evening, DFW went into a meltdown due to thunderstorms. I had a friend connecting at DFW on American, and they diverted his (and about 40 other flights) to Austin. He didn't get where he was going until the next evening. (I had to play travel agent for him to try to find alternative flights. AA refused to rebook him on United through ORD which would have gotten him home earlier, but it could be that those flights were also full by the time he got through to someone for rebooking)