#1 - Court geometry
Court geometry last night was at the heart of the battle. Both teams set up their players in precise positions to ensure their stars had the most space to attack the rim. For example, to start the game, the Celtics set up Kristaps Porzingis against Cason Wallace so he could ignore him and remain close to the rim.
However, to counter that, all the OKC off-ball players moved to the opposite side of Cason Wallace and Porzingis. As a result, there was a lot of space on the left side, where only Wallace and Porzingis were.