The three Walking Dead movies starring Andrew Lincoln as Rick Grimes may be the best explanation for Michonne's exit from the series in season 10. Last week, it was reported that Danai Gurira, who has played Michonne since The Walking Dead season 3, will leave during the show's tenth season. During her final season, Michonne will only appear in a "handful of episodes".
Five episodes into The Walking Dead season 9, Rick was supposedly killed after triggering an explosion on a bridge. Rick's plan was to sacrifice himself and destroy a bridge in an effort to prevent a hoard of zombies from crossing. In the aftermath of Rick's "death," The Walking Dead skipped forward by six years and introduced a new threat: the Whisperers. Many of the characters have moved on with their lives, including Michonne, who has taken leadership of Alexandria.
Given Michonne's new responsibilities, it would normally seem unlikely for her to leave Alexandria behind - unless she has a good reason. According to the original report from The Hollywood Reporter concerning Gurira's exit, the actress is expected to return in the three Walking Dead movies, though this hasn't been officially confirmed. However, this move does seems like the most logical way to finish not only her story, but the story of the entire Grimes family.
Rick survived thanks to Jadis, who had a helicopter take him to a currently unknown location. Where Rick ends up will clearly be the focus of The Walking Dead movies. Given their impending production, it's possible that Michonne's departure in The Walking Dead season 10 is connected; it's possible that she will somehow discover that Rick didn't die that day on the bridge and go looking for him with both of her children.
If a Grimes reunion is in The Walking Dead's near-future, a lot needs to happen first, especially on Michonne's side. Presently, everyone believes Rick is dead and has no reason to suspect otherwise, so something major would have to occur in order to sway Michonne into leaving. This could happen in the form of a message from Rick, or through the sudden return of Jadis.
Since Andrew Lincoln has confirmed that he has no plans of ever returning to The Walking Dead TV series, it would make perfect sense for a reunion in the films to serve as the conclusion of their story. Michonne, Judith, and R.J. can finally reunite with Rick in the movies, and the final film can tie up any remaining loose ends with these characters.
The Walking Dead continues Sundays at 9pm on AMC