Pretty much this.
We gave him the captaincy to retain him last time round, it didn't work, and i suspect we didn't make anywhere near as strong an offer to keep him this time round given we now have Shinnie (captain in waiting) and McLean.
Certainly a distinct lack of "we want to build a team around him" chat from Deek this time, and i'm guessing Jack is injured for the rest of the season hence why the decision has been reached early - good for both us and him to conclude quickly.
If he does choose to go to Petulant Pedro, he knows how that would be viewed by Dons fans, so how Jack responds to Sevco will be a fairly good indicator of his level of respect for a club that he has spent his entire career at.
As i said before, innocent until proven guilty, so let's just see.