I've never heard of being "bumped" per se however I have had the cruise line call me on numerous occasions offering clients an upgrade. IE: Had 4 people in 2 balcony cabins on Princess once. Princess called and offered an upgrade to a family suite (a huge upgrade). I knew Princess must have needed the two cabins my clients were booked in pretty badly to offer such a huge upgrade. It was my clients' option to accept or decline the upgrade (a no-brainer in this case). In 9 years I have never had a client "bumped" without their consent. I do not believe the cruise line is able to do that once you are booked.
To answer Joe's question, the cruise line can capacity control the occupancy of each category. In the case of aldesonia, it sounds like they were TRYING to book but the cruise line was holding the remaining cabins in that category for groups of more than 2. They CAN do that if they want to in order to save room for the larger parties.
Happy Cruising,
Michael