https://screenrant.com/why-pauley-perrette-left-ncis-retired-acting/

Retired actress Pauley Perrette's Abby Sciuto was a beloved part of NCIS, but her exit from the show is a result of BTS problems with a co-star.

Here's Why Pauley Perrette Really Left Her Role As Abby On NCIS (https://www.nickiswift.com/1484430/why-pauley-perrette-left-role-abby-ncis/)

Pauley Perrette was a fan favorite during her 15 seasons in the role of Abby Sciuto on "NCIS," but behind-the-scenes unhappiness caused her to leave the show.

Did Pauley Perrette and Mark Harmon get along? (https://www.quora.com/Did-Pauley-Perrette-and-Mark-Harmon-get-along/answer/Jon-Mixon-1)

Q: Did Pauley Perrette and Mark Harmon get along (https://www.google.com/search?q=Did+Pauley+Perrette+and+Mark+Harmon+get+along%3F&oq=Did+Pauley+Perrette+and+Mark+Harmon+get+along%3F&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIHCAEQABiABDIICAIQABgWGB4yCAgDEAAYFhgeMggIBBAAGBYYHjIICAUQABgWGB4yCAgGEAAYFhgeMg0IBxAAGIYDGIAEGIoFMg0ICBAAGIYDGIAEGIoFMg0ICRAAGIYDGIAEGIoF0gEJMTExOWowajE1qAIMsAIB8QWmVGMVIj1FPg&sourceid=chrome&ie=UTF-8)?

Apparently they did for a number of years as Harmon was the star of NCIS and Perrette, while the series breakout character, would have still had to maintain at least a cordial relationship with him to have remained on the series.

This seems to have changed in early 2017 when Perrette claimed that a co-worker was bitten by dog that Harmon had brought to the set and then allowed to roam around off-lead. The animal was removed from, the co-worker was treated and compensated for their injuries, and most of those involved seemed to believe that the matter had been settled. Perrette however was no satisfied even though she was not herself a victim in the attack. She began a “campaign” for more to be done and unfortunately for her, she seems to have gone a little too far as she was asked to leave the program. She has never returned to the program; nor have either her nor Harmon ever “cleared the air” about the events leading to her departure.

Perrette later claimed that “physical violence” on set is why she left in addition to the dog attack. However since didn’t a grievance with SAG-AFTRA (the actors union), nor any police reports, it’s virtually impossible to determine if she was/is telling the truth. She apparently has no relationship with Harmon any longer, who retired from the program four years after the events in question, although he remains a producer of the series.

The two in “happier times”:
https://qph.cf2.quoracdn.net/main-qimg-75e0d867216998daf94509db3d8a6796