It wasn't exactly clear why the trio were leaving the series, but it was understood that D'Onfrio would be leaving sometime during the series' upcoming ninth season, where he would hand over the top detective duties to Jeff Goldblum's character. It was also earlier reported that Julianne Nicholson was leaving the show, and will be replaced by Saffron Burrows.
It was theorized by the trade that the move could signify a shift in tone for the series, in handing the reigns over to Goldblum, whose lighter demeanor falls more in line with the USA Network's branding as displayed in shows like Monk, Psych and Burn Notice.
While D'Onofrio would be leaving the series, it was said that it would be left open for the actor to return in a guest-starring capacity in the future.
Sources: The Hollywood Reporter
BEST OF THE WEB