There have been some hefty thunderstorms in the area. Wednesday night/thursday morning was particularly bad for Northern Illinois as many trees were down and power was out to a good number of people. Fortunately the heat wave blast is gone and the temperature is cooler for the time being, which means fewer thunderstorms.
Overall I'm guessing this reaction for United is because a lot of business travelers that were doing day trips from ORD
probably would miss their meetings because of the mess of United operations. Rather than having to deal with hundreds of standby passengers, they are probably trying to get rid of some of the people since the load factors are really high on the flights as is and there isn't space to get everyone out in a timely fashion. When you have full flights and massive cancellations, there is no easy way to get things moving again in a timely fashion, so they are trying to get people to rebook. I think it makes sense. But in reality Chicago isn't in a disaster situation like what normally causes these policies.
If you have never designed an airplane part before, let the real designers do the work!