For this (and seat shenanigans reasons) I *always* print a BP from the kiosk at the airport, even if I have a mobile BP on my phone which I intend to use.

With that said, you should not have been removed from the flight until you actually miss the boarding window (despite what the UA algorithms may falsely calculate) and should complain. You could also submit a complaint with the DOT if they don't give you the proper compensation.