Pure speculation follows
In terms of tennis philosophy, it always struck me that the Agassi-Djokovic partnership seemed a mismatch. While Becker came in and his skillset to help Novak involved the serve, and playing clutch on the big points, where Boris often would have been aiming to shorten the points and therefore had much to teach, Agassi is a back of the court first strike merchant. I felt the opportunities for disagreements were large - Novak's superior flexibility and movement would mean he would be prepared to take steps back in the court in favour of overall pointsconstruction, where Agassi would get right on the baseline and trust his timing to go toe to toe. Additionally, Novak's willingness to grind players down and take their legs away contrasts with Andre's power hitting and eagerness to impose himself through sheer ball-striking, even if this meant shorter points. Novak constructs and plays very differently from Agassi, whose talents for timing and hard-hitting aren't really things that strike me as very coachable when we're talking about giving a seasoned, top pro the 1-2% that keeps them ahead as the very best.
Add to this the key facts that Novak has in all honesty not recovered from this injury, and has lost motivation in part as a result of this, and it seems the timing of this is the best for all concerned. Agassi has always struck me as someone who was only going to come back to the game for specific reasons, and it doesn't sound like he was being respected as a coach, for whatever reason.
