On a recent jetBlue flight from Boston to West Palm Beach the gate agent announced that there was a "critical weight imbalance" and seven volunteers were needed to move from the first five rows (Even More Space seats) to empty rows elsewhere. The agent started by offering a travel credit of $100, then $200, then $300, at which point there were enough volunteers. I held out until $200, but got the $300 credit anyway, My wild guess is that the flight had more checked luggage than usual; checked luggage is weighed. If the flight from Costa Rica was completely full, moving passengers around the plane was not an option.