I have no knowledge if this is or is not related to the MAX grounding. However, I am surprised about this cancellation. The 5pm bank of flights at IAD is essentially the only bank left. It’s the bank that connects to all the transatlantic flights, which are very popular in the summer. I would guess this flight has a 90%+ LF during the July.

Assuming UA had to cut flights due to aircraft availability, this is not one I would have thought would be on the chopping block.