UA flies Members of Congress and other high-profile government officials all the time. Jackson Lee likely flies this route weekly when Congress is in session. Same for other Members for their respective routes.
I consider it unlikely that UA would boot a passenger and force a DG, knowing that it would cause a scene which is problematic for both UA and Jackson Lee as it did. Much more likely is that the passenger inadverdently cancelled or the UA app glitched. While it was possible to reinstate the ticket, there was no F seat, thus Y.
If UA had wanted to do this, it presumably knew that Jackson Lee was flying and could have handled all of this behind the scenes. Another signal that this was a routine error which, on a percentage shot, lands on a well-known passenger.