I don't follow them particularly closely, but from what I can gather the catalyst for change was the coaching style. (The squad didn't change but they managed to keep Reynolds on the park for extended periods which helped a lot - unfortunately for us).
I subscribe to the theory that Madge was a detail man, and had the team drilled so hard that they were mentally fatigued. Madge has ways that he expects the team to play, even if that means jamming a square peg into a round hole. This sort of coaching works really well when a team has been lacking in detail, and allowed to play whatever Is in front of them. You don't coach all of the footy out of a team in one or two seasons, hence they won a premiership early in Madge's tenure. Eventually though, the mental fatigue effects their footy, and they stop enjoying themselves, which I think explains the poor results at the end of Madge's time there.
The muscle memory was still there when Seibold took over, but he loosened the reins, and gave them the freedom to play off the back of Cook's runs. Cook would look wide, and the rest of the team pushed up because they knew he would. Worked especially well for someone like Cody Walker, who is a gifted natural footballer.
Change in style having a positive effect is a phenomenon that I see in patterns in the game. It's particularly evident when a rookie coach is given the reins after a coach is sacked mid season. Think roosters when Freddy started or knights when Rick stone took over. Neither of those two ended up being great coaches, but the players responded to the change.
How this applies to Seibold at Brisbane I have no idea. Last year Wayne had given the team a lot of freedom to play, with varied success.