Why Pauley Perrette's Abby Sciuto Left NCIS After Season 15

screenrant.com:

Abigail Sciuto (Pauley Perrette) was a beloved character on NCIS up until season 15 when she suddenly left the show. Abby’s happy goth personality won her an army of fans throughout her time on the CBS drama, many of whom still miss her to this day. She will always be remembered for her pigtails, chunky platforms, and her beloved Caf-Pow drink—as well as her brilliant brain as a forensic scientist.

Abby was not only a hit with viewers, but also with her NCIS co-workers, including special agents Tony DiNozzo (Michael Weatherly) and Timothy McGee (Sean Murray), and former team leader Leroy Jethro Gibbs (Mark Harmon), who was always a father figure. When Abby needed her next Caf-Pow fix, Gibbs—replaced by Gary Cole's Alden Parker—was there to hand it to her (in exchange for important forensic information). The characters’ close relationship on-screen appeared to signal a bond in real life between actors Harmon and Perrette. However, this was not the case.

Related: NCIS: Why Michael Weatherly Should Not Return To Replace Gibbs

To her NCIS fanbase, it was the end of an era in 2018 when Abby Scuito left NCIS after 15 years on the procedural drama. Initially, it appeared to be on good terms, but it was soon apparent that there was more to the story. It was revealed that Perrette and Harmon had a major falling out on set in 2016, and this led to a complete breakdown of their personal and professional relationships. What happened afterward led to Abby leaving NCIS—almost as unexpected as the shocking death of Caitlin Todd.

According to reports, the fall-out began after Mark Harmon—also a producer on NCIS—brought his dog, Dave, onto the set (via The Bark). The dog is said to have bitten a member of the NCIS crew who later needed