I fly out of OAK regularly. I recall volunteering once on an oversold Denver flight, and they didn't use me. In this case, they were worried about my making a connection, where the others they did bump were ending in Denver. This does not seem to the situation with you, though.

I'm convinced frequent flyer status does play a part in bumping with UA. I was bumped 8 times last year, and can't recall a flight that they did not use me when they needed someone. I'm at the 1K level though, and I think that moves me to the top of the list.

Do you have any frequent flyer status with UA? Maybe the others were higher tier frequent flyers?