It really looks like a product of the generally poor defensive discipline that the Spurs have as a team. It's gotten better as the season went on, but still within the typical "young team" realm.
It's compounded with another of my pet peeves: since our guards like to "leak out" ahead, they leave Wemby behind to grab the board and also push the ball up, and he's not so good at it - leading to steals and turnovers, but also to him defaulting to "starting the play" and going into those combo-dribble moves that seldom work.
I'd like better communication/duty sharing from our guards so that one of them always tries to grab the rebound (kinda like Westbrook does), AND also tries to start the play. A soft screen-and-roll with Wemby could be so powerful...... If they'd just do it consistently!
I looked up more numbers, and this could be a stretch. I am putting in defensive put back numbers (when the Spurs are on defense and the other team has a put back).
The Spurs ranked 3rd best in the league in defensive put backs per game at 5.4 possessions. This looks great until I realized:
1) The Nets are #1 at 5.2, and not coincidentally, the worst defensive team by FG% in the league. The other team scored on the first try, so there are no put backs. Not coincidentally the Nets are also last in DReb, as there are just less of those given they aren't that good at forcing misses. This isn't an issue for the Spurs as we are #4 in the league in defensive FG%.
2) The Spurs are #1 in defensive rebounds, so shouldn't we be #1 in defensive putbacks as well? The Knicks are #6 in DFG% and #10 in Drebs and yet #2 in putbacks allowed at 5.4, with 4 players having a DRB% > 20%, and another 5 >15%. Hornets are #16 and #8, and yet #4 in DPB possessions, with 3 players having a DRB% > 20% and another 6 >15%. As a comparison, the Spurs have 1 player >20% (Wemby with a league leading 33%) and another 4 players >15%. Our good friend OKC is actually relatively poor in this area ranking 22 in the league in DPB, even though they are #5 in defensive rebounds but #1 in DFG%, they have 3 players > 20% and another 2 >15%.
3) However, OKC is #3 allowed in PPP in DPBs, at 1.02. The Wolves and Cavs are #1 and #2 also at 1.02 and not surprisingly, they both have more than 1 rim protector (Gobert, McDaniels, Reid for Wolves and Mobley, Allen for Cavs). I take it OKC just hack the living crap out of people under the basket without getting called. The Spurs are middle of the road, ranking 17th in the league at 1.11 PPP on DPBs. A bit surprising given that both Wemby and Korndog are pretty good at protecting the rim.
To your point, the Spurs should put some emphasis around securing the rebound once Wemby challenges a shot. It's all nice when we go out for fastbreaks as we have young legs and players who are devastating in the open court (Castle, Harper, Keldon and of course Fox. But even Wemby and Vassell are great). but if we can't secure the rebound for those fastbreaks, it is all for naught. We are already the best defensive rebounding team in the league, and hope we can take a bit of that pressure of Wemby and not rely on him to challenge the shot, get the rebound, start the break and sometimes finish the break.