I was surprised when I flew United from South Bend, IN to OHare last Tuesday.
I had my tickets for weeks, and this was my return flight.
When I went to the main ticket counter, I was told there was no assigned seating, as the flight was overbooked!

(How comforting)
Luckily I was by myself and I knew from flying in that this plane only held 26 passengers.
I was told they would call me before boarding. (I was checking in 1.5 hours early and there were only a few people in front of me who also checked in.)
As people were arriving, I could overhear they had assigned seating.

About half an hour before flight take off they were asking for SEVEN volunteers, due to the overbooking.

(Out of a plane that only carries 26, I thought that was a lot!) United was giving free RT tix to the volunteers. hhhmmm... I even stood in line to check it out.
What I thought was odd, only four people were in front of me. The first passenger was flying to Portland and United would not change her flight. They refused saying she and they were better off the way it stood.
One guy, and only the second in line and being waited on, did have a family, was changed, and then the plane was freed up! I thought he only had four in his party (not like I was stalking and counting his group or anything).
After him, they called my name and around five others. I think us who first checked in were not given assigned seating!
I would have thought United would do the opposite. When overbooked, ask for volunteers, but seats would be assigned on a first come first serve basis. Apparently not.
My luggage had the OHare routing tag. It also had a PENDING tag!
DH thought maybe all passenergers on that flight had a pending tag on their luggage.
The *new* assigned seat was not a window seat, as I had requested and was originally assigned. But it wasn't any big deal.
I just have never run into an overbooked flight and thought this was handled odd. But maybe not. Maybe this is the norm.