Maybe there is no "other" reason and the reason is simply that 3 ambitious players want to further their careers.
Payne has moved to a club in the championship. No doubt finances played a part in the decision but it is pure conjecture that it was the sole decision. Unlike, say Barry Corr, he is still moving up a division so it still makes sound footballing sense to move to Huddersfield.
Brentford for me is an excellent choice for Dan. He has a real opportunity to be playing first team football at an established Championship club. Maybe he could go to a "bigger" club but he may then find himself on the bench.
It was indeed widely assumed by many that Worrall would go back up north but I do not recall Worrall himself saying us much. Again Millwall looks like an excellent move. They took awhile to adapt to life in League 1 but finished the second half of the season strongly, made the play-offs, and must be one of the favourites for promotion next year. He also doe not need to relocate.
I think Worrall moved simply because he wasnt getting regular foobtall, cant blame him for that and can leave that at Browns door, but he is the manager .
The other two moved because they had a chance to play in the Championship so no blame on our part other than not sorting out Paynes contract beforehand.