Replacing Ibaka and Gasol with Boucher (playing more) and Baynes is a big reason for this start. Even a declining Gasol was a smart defensive player and a good passer. The front court loss is huge, and it's making an impact on D and on offense.
Any time you lose a top 5 player (Kawhi) + players who could start on a championship team (Green, Ibaka, Gasol) and not replace them with even an established NBA rotation player aside from Baynes, or a high upside young player, then it's going to be tough to keep winning. It is still early, but no way this team succeeds beyond a 1st round exit with the current front court. Plus, Siakam is looking like not quite an all star anymore, and a lot of the team's success was riding on him taking that next step. He's actually regressed since the bubble.