I think WHY airlines overbook is pretty obvious- to make more money.
This video says they have tons of secret research to help them figure out when they can and by how much... but obviously they get it wrong sometimes.. normally denying access to 9 out of every 10,000 flyers.
It also says if you don't wanna be chosen to be kicked off, it's as simple as having a frequent flier account. You don't have to use it much, it doesn't have to be a credit card... but have the account, check in to your flight ahead of time, and get your bags in early- you'll be set.