Join Date: Nov 2007
Location: Philadephia, PA
Programs: AA Platinum, Marriott Titanium/Lifetime Titanium, Hyatt Globalist, UA Silver, Hilton Gold, Hertz 5*
Posts: 477
"Denied boarding involuntarily" has a specific meaning that is linked to a United States Department of Transportation policy and requires that the flight be oversold. That doesn't apply to either a mechanical issue or a cancelled/nonexistent flight. It might apply to United's bumping you if the delay was sufficient and US didn't mess up when adding you on the United flights, but then United would be paying it I believe since they made the decision to bump you.
Did you ever look at your flight on US Airways' site? I'm wondering if your return flight got cancelled as part of the scheduled changes from the US/AA merger and you got rebooked on something else. Odd that would happen without US notifying you, assuming they had a contact method to reach you.
How would United have been able to tell you your return flights on US/AA were cancelled? The original itinerary was all US or AA, right, or was it Expedia piecing together other airlines as well?