Part of his jump in rebounding is how the Clips are defending Lillard. Jordan has been trapping Dame out on the perimeter, which pulls him away from the basket, which makes it easier for Plumlee to get rebounds.
There are many factors at play, but Plumlee's offensive rebounding has not increased dramatically in the playoffs due to Jordan double teaming Lillard. His offensive rebounding has remained very close to what it was during the regular season.
Regular Season:
ORB% = 10.4
ORB/36 = 3.5
Playoffs:
ORB% = 10.8
ORB/36 = 4.0
So, a very minor, almost negligible, uptick on offensive rebounding.
Where is rebounding has shown a massive increase is on the defensive end.
Regular Season:
DRB% = 22.2
DRB/36 = 7.4
Playoffs:
DRB% = 33.1
DRB/36 =
12.2
That's a HUGE jump in defensive rebounding.
In raw stats, Plumlee has seen his playing time increased by nearly 4 MPG compared to the regular season. That helps his rebounds per game, but does not explain the huge increase in DRB% and DRB/36. As mentioned, the Clippers are a poor rebounding team (28th in the league in both team ORB% and team DRB% during the regular season). I think part of that is they use their best rebounder to set on the ball screens out beyond the 3-point line. With Deandre Jordan that far from the basket, and a complete non-threat to score more than 3 feet from the basket, that allows Mason Plumlee to sag back and Hoover up a shit ton of defensive rebounds.
Throw in the fact that the Clippers are missing a lot more shots per game in this series than the regular season and there are more defensive rebounds available. During the regular season the Clippers averaged 44 missed FGAs per game. In Game 1, the Clippers as a team shot .538 FG% and only missed 36 FGA. It's not surprising that Plumlee grabbed his series low of only 5 rebounds (3 DRB) in that game. There just weren't that many defensive rebounds available in that game. Now look at Games 2 - 7:
Game 2:
LAC missed FGA = 47
Mason Plumlee TRB = 10
Mason Plumlee DRB = 8
Game 3:
LAC missed FGA = 52
Mason Plumlee TRB = 21
Mason Plumlee DRB = 18
Game 4:
LAC missed FGA = 54
Mason Plumlee TRB = 14
Mason Plumlee DRB = 9
Game 5:
LAC missed FGA = 50
Mason Plumlee TRB = 15
Mason Plumlee DRB = 11
With the Clippers missing an average of almost 51 shots per game in Games 2 - 5, there are simply a lot more defensive rebounds to be had. Combine that with more minutes played and the fact that the Clippers are a weak rebounding team and you see Plumlee's huge uptick in total rebounds.
To answer the OP's question, no I don't think this is sustainable over an entire regular season. I'd love to see Plumlee prove me wrong and average over 10 RPG next year, but right now he's taking advantage of a unique set of circumstance (same for his ridiculous assist totals in this series), but he's an intelligent player with a great motor and good athleticism, so anything is possible.
BNM