maybe they were on "the list" before you... did you get up as soon as they announced that they were looking for volunteers?

also, with regard to your "who gets bumped" question I think that often times it has to do with:

1) who can they accomodate on their airline on a different routing. Happens more at a hub where they'll pawn you off on one of their connecting flights.

2) large blocks of people. If there is a family of (4) travelling together and they're eager to take a bump, that is much easier for the airline to deal w/ than (4) individuals. It also frees up the space faster.

3) people that they know can be reaccomodated on the same day. I know you were flying direct, but if you were flying some little podunk town w/ limited service then it would be difficult for them to get you another itinerary...

I'm sure others will other reasons...