It is an unfortunate reality in baseball that teams occasionally lose players that were fan favorites to free agency or that these players are dealt away towards the end of their careers.
Many of these players get a chance to return to the stadium that they spent a large portion of their career in, and they get another chance to play in front of the fans that cheered them on for so many years.
Some teams choose to honor their former players when they make their return, even if they now play for a rival. Fans have had mixed reactions to their former players coming back. Certain players get booed when they come back to their former home for the first time.