It's been quoted that Collin was on something like £3000 a week, it's a lot of money for a goalie who roots himself to his line, has poor positional sense and has problems getting down to low shots fast enough. I suspect Nolan has been told if he wants his 25+ goal a season striker, he can't be spending that much on an average keeper?
Interesting, if the situation is as it reads. Maybe the club were not willing to meet Collin's ongoing wage demands even though Kevin Nolan et al personally wanted him to stay?
In the official press release there were references to the decisions being made "as a management team", so maybe there was a lot of push and pull between the football management side and the club management side, leading to these decisions.
Chairman sets the budget. If Collins wages are over the wage cap or something, then its a simple decision.
if you are using money for a striker, you have to take it from somewhere. Maybe they thought they could get an equally good keeper for less, and use the difference for a better striker?