Maybe the KC Chiefs traded Tony G so they got something for him before he retires or maybe he just wanted out to go to a team that is on the up, rather than stay with a team that does nothing every year! Being a future Hall-of-Famer means very little to the players old team when the player is near the end of their career. Joe Montana, ended his career as a Chief.
Emmitt Smith was a Cardinal (re-signed with the Cowboys to retire a Cowboy).
Deion Sanders, retired as a Raven.
Brian Dawkins, will retire as a Bronco after 13 years service for the Eagles.
The list can go on, sometimes it's just time to move, for your career or the future of the team.
"The harder you work, the harder it is to surrender." Vince Lombardi