There are knock-on effects from the MAX grounding... for instance, they initially were scheduled on a number of West Coast-Hawaii flights, but those flights haven’t been canceled altogether. Instead, other airplanes are swapped in, which come from somewhere, and those flights need to be covered be other equipment, and so on.

Eventually, something has to give, and it’s very unusual for UA to be pulling service so late during the busiest season of the year. Effects of the MAX grounding are a logical explanation.