Well, the only information I can find is that United Cargo indicates "operations." This can mean that they are simply short on planes and need it more on a different route. I usually find they're fairly helpful if I call and ask.
If you're on this flight, you can reschedule through ORD or DEN, with many more options through the former. It'll add a good hour and a half to your travel time with that lovely walk from B or C to F at ORD. (Actually, it always ends up being a sprint for me.)