Kevin Costner was just as surprised as anyone when his wife of 18 years filed for divorce. Some fans speculated it was due to his commitment to Yellowstone, but his forthcoming project, Horizon, may have played a bigger part.

While Costner continues to work out the details of his split from Christine Baumgartner, here's the real reason why she "blindsided" him with the divorce.

'Horizon' may have been the last straw in Kevin Costner's marriageCostner's shocking divorce is not playing out as he intended. When Baumgartner officially submitted the divorce papers, sources claim that he had no idea she wanted out of their marriage.

An insider told OK Magazine that the Yellowstone star is devastated by the potential disaster he's facing in his career and personal life. Despite the challenges he is facing, Costner is doing his best to navigate the difficult situation.

The source also divulged why Baumgartner ended her marriage so abruptly. Although Costner took a step back from Yellowstone to help save his relationship, his commitment to Horizon ultimately sealed his fate.

"Kevin paid her lip service by demanding a more favorable shooting schedule on Yellowstone," the insider dished. "But then he threw gasoline on the fire by using that time to focus on his Civil War project.

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The divorce news came amid rumors about Costner's ongoing feud with Yellowstone creator Taylor Sheridan. The two reportedly butted heads after Costner cut his season 5 shooting schedule down to a single week.

Kevin Costner is still working hard on his 'Horizon' project amid his shocking divorceDespite everything that is going on in his life, Costner remains dedicated to his Horizon project. The movie is currently in production and is set in both the pre and post-Civil War times.

With filming underway, Costner recently took to social media to promote his upcoming epic. In a post on Instagram, the Yellowstone star shared a photo of a director's chair with "Horizon" written on the back.

"We're deep into the production of #HorizonFilm, and, I have to tell you, I haven't felt this way about a movie since we were making Dances With Wolves," he wrote.

An inside source, meanwhile, told Page Six that Baumgartner did not approve of Costner's busy work schedule. The insider claims that Costner did not spend a lot of time with his family and was obsessed with his various projects.

Costner first announced Horizon at the start of 2022. The project is the first time Costner is serving as a director since his 2003 film Open Range.

'Yellowstone' star could lose a lot in the heated divorce battleAs Baumgartner and Costner prepare to navigate a challenging divorce process, she isn't pulling any punches.

A source close to the situation claims that Baumgartner won't be satisfied until she gets some of his fortune.

"He's worth a quarter of a billion dollars, and she had the tiniest engagement ring in Hollywood," the insider stated. "He'd think nothing of being away for months at a time on the Yellowstone set — or other projects — if he thought it would cement his status as one of Hollywood's biggest stars."

As Costner navigates the challenging path of divorce, he will simultaneously be parting ways with the show he made significant sacrifices for. Insiders reveal that his frequent interruptions in production, caused by scheduling conflicts, ultimately led to the premature conclusion of the series after Season 5.

Production on season 5 of Yellowstone was supposed to begin earlier this year. Filming, however, was delayed amid rumors of Costner's drama.

In light of all the reports, Paramount recently confirmed that Yellowstone will end after season 5 airs later this year. Costner, meanwhile, recently confirmed that he and Baumgartner are calling it quits after 18 years together.