I'll try to keep this short, but bear with me. I think there's three (non-exclusive) hypotheses here: (1) by random chance, opponents are scoring more against the Celtics; (2) opponents are making more 3's because the Celtics defense has slacked off (or they've made a conscious decision to defend the rim and give up more looks at the perimeter, which I personally think goes against the very core of Mazzulaball, but I'll include that here because the two are indistinguishable in the data I'm using); (3) opponents are making more 3's because they're taking more 3's, in an attempt to out math the math kings.
So, two points. First, as I said in my earlier post, Celtics opponents have gone from attempting 16 wide open 3s in October to 20 wide open 3s in January. That could just be because they're attempting more 3's overall, and indeed as
@Euclis20 posted (thanks for pulling those data), opponent attempts have gone up from ~35/game to 39/game - consistent with hypothesis 3. As a percentage of total 3 PAs, opponents' wide open 3's have gone from 46% to 52%, consistent with hypothesis 2. (Actually, it shot up to 52% of all attempts in November and has stayed there in December and January). The point stands: compared to the beginning of the season, opponents are taking 4 more open 3's per game, which represents a structural change in how opponents are strategizing against the Celtics and/or how the Celtics defense is working right now.
Second, we can do a very simple linear regression to look at the change in 3PA, 3P, and 3P% since the beginning of the season:
View attachment 94729
The circles are the observed 3P% by game, the line is the trend over time. The distance from the trend-line is a more quantitative estimate of game-to-game shooting variance
@lovegtm is talking about (all else being equal), and indeed the Pelicans were lucky. At the same time, opponents' 3P% has gone up about 6 percentage points from the beginning of the season. That's not random, consistent with hypothesis 2. The attempts have also gone up, about 3.7 more per game since the beginning of the season, consistent with hypothesis 3.
Put it all together: opponents are taking almost 4 more 3's per game now than they were at the beginning of the season, they're making them about 6 percentage points more, resulting in about 3.6 more made 3's per game. And, just looking at January, MIN, OKC and NOP were pretty lucky to make the number they did. So, I kind of think it's a little bit of all three arguments. To say nothing of whatever's going on with the Celtics' offense.