Just say you have had that star player at the club for a number of seasons now and you have done things to keep him playing on at higher increasing levels, there comes a time where that player begins to decline with age/star rating and can't keep up with the team/level. A midfielder or striker that scores 30+ goals a season? A player that gets many 8's or 9's in ratings resulting in plenty MOTM performances. With these sort of things happening, can managers get attached to players that have done great things at the club?
I once recently sold a striker I bought as a 18 year-old in Level 2, and used plenty of tokens/rest packs developing that player, but at 29 years old and scoring over 250 goals in all competitions there does come a time when you have to move that player on, even though you would like to keep him until maybe he retires to see how far he develops in the career.
I also had a Midfielder called Yossi Benayoun in the lower levels with a free kick specialist and he was scoring more goals a season than the strikers were. When I sold him around the beginning of level 8, the new manager changed his name...