From the quote in the footnotes: "I do find bunching at 280g after 2010 in case management data from the Executive Office of the US Attorney (EOUSA). I also find that approximately 30% of prosecutors are responsible for the rise in cases with 280g after 2010, and that there is variation in prosecutor-level bunching both within and between districts. Prosecutors who bunch cases at 280g also have a high share of cases right above 28g after 2010 (the 5-year threshold post-2010) and a high share of cases above 50g prior to 2010 (the 10-year threshold pre-2010). Also, bunching above a mandatory minimum threshold persists across districts for prosecutors who switch districts. Moreover, when a “bunching” prosecutor switches into a new district, all other attorneys in that district increase their own bunching at mandatory minimums. These results suggest that the observed bunching at sentencing is specifically due to prosecutorial discretion"

If you had to work to parse it like I did, the TL;DR is that a minority (30%) of prosecutors are responsible for the "bunching" and it seems they intentionally are choosing to prosecute more cases at this level.

So, there may be police falsifying evidence, but the paper couldn't detect that and it seems more driven by prosecutors choosing to do "bunching" to get the amount of crack to hit the minimum.

IANAL so not sure what bunching means. Maybe instead of charges from separate arrests, they group them together as one large case?