I was on a flight EWR-SFO recently. The flight was booked full. Before boarding started they asked for volunteers for a confirmed seat 2 hours later on a United flight in addition to a $500 travel cert. The reason given was that there was a weight and balance problem. Don't know if they got any volunteers, because I boarded in a few minutes, but the GA did board the plane before the door was closed and asked for another volunteer before we could leave. Some one - volunteered so we were able to go. The person looked like they weight 120 lbs.

My question is what could cause a weight and balance problem on a 737-800? The only thing out of the ordinary of this transcon was a head wind which increased flight time about 45 minutes. Could this be the cause? Any ideas? Maybe lots of freight?