Getting "bumped" off a plane usually really pisses people off, or it makes them quite happy. The later is likely a voluntary bump and the former an involuntary bump. You know when something is involuntary is just can't be good; indeed the case here. Scenario: Your flight is oversold by 10 seats. The gate agent asks for volunteers to give up their seats in exchange for some denied boarding compensation (free air credit, ticket, upgrade etc) and put on a later flight. Lets say they can round up 9 good ol' folks to accept the voluntary bump- they walk away happy with some freebies in hand; that leaves the airline now 1 seat overbooked. The airline then initiates an involuntary bump; some lucky passenger will be pulled off the flight and accommodated on another flight - whether they like it or not. I've seen ordinary calm and pleasant passengers become hysterical in a matter of minutes once they've found out the ticket they have in hand is useless because of an involuntary bump.
It is here, the involuntary bump where the federal government steps in and monitors what's going on. Mandated reporting of INvoluntary bumps by the airline can show trends and whether the airline is just overbooking flight to frequently. The government also specifies mandatory compensation for INvoluntary bumps, whereas voluntary bumps are negotiated (in theory) between the passenger and gate agent.
INvoluntary bump compensation is being discussed by the Department of Transportation, specifically whether the compensation levels should be increased. The press release is here. Essentially 5 proposals, most to increase a displaced passengers compensation to a higher amount for being inconvenienced. DOT is accepting comment on the proposed changes.