According to the source, things have turned sour for the pair -- who announced their decision to divorce on Sunday after 10 years together -- after Cavallari requested primary custody in divorce documents.
"Kristin and Jay planned to keep everything amicable, but that's not the case anymore," ET's source says. "Kristin filing for primary physical custody upset Jay. He loves his children more than anything and thinks the couple should be reasonable and split custody."
"Kristin has made comments about possibly moving back to L.A., which is also a concern of Jay's, as home for the family is currently Nashville," the source adds.
In court documents obtained by ET on Monday, Cavallari listed the date of separation as April 7, citing "marital misconduct" and "irreconcilable differences" as the reason for the breakup. She also claimed that Cutler "is guilty of such inappropriate marital conduct as renders further cohabitation unsafe and improper," adding that "any misconduct alleged or implied against her in the complaint for divorce was in response to and/or as a result of misconduct on behalf of the Husband."
Cavallari is asking for primary physical custody and seeking child support for her and Cutler's three kids -- Camden, 7, Jaxon, 5, and Saylor, 4. She's requesting that Cutler be allowed visitation, and that the former NFL star pays for their children's health insurance and maintains a life insurance policy with her name listed as the sole beneficiary.
Cutler, meanwhile, is requesting joint custody of their children and equitable distribution of the marital assets, stating that he has always been "the available at home parent and primary caretaker of the parties' minor children."
Cavallari denied Cutler's claim in her filing, alleging that she has been "the primary residential parent" and that she is "a fit and proper person to be named Primary Residential Parent."
The court docs also reveal that a temporary restraining order was filed, which is said to be a standard in divorces in the state of Tennessee. The restraining order prevents Cutler and Cavallari from disposing or concealing marital property, harassing each other and relocating the children.
See more in the video below.