It's not that rare. It's not something that happens every season, but it's not rare. All it takes is interest from a studio. I don't really feel like trolling through old anime lists for more examples, but I'm sure there are plenty.
Hatsukoi Ltd. is another example I can think of from Jump. The anime began about a year after the manga ended. JoJo's technically is not completed, but the 2012 anime adapted material that serialized over twenty-five years ago.
I don't think is that weird for JUMP series to get animes after they have finished.
As Kaiten mentioned Jojo and Hatsukoi both had finished, same with Ring Ni Kakero a manga that was in the 70s. Then you have this other manga made by Buronson I think, Keiji which got an anime right now. Also Level E.
Then you have "Jump's survivors" series that made it to have a long run but weren't super popular in the TOC and weren't super hot sellers either (Maybe that's why they didn't get adapted earlier) like Hoshin engi -it got adapted in 1999, Hoshin Engi finished in 2000-, Black Cat, Buso Renkin.
Kochikame got an anime like 20 years after serialization.
Basically if its not a big hit it might interest an anime company that sees potential for niche fans of the series.