Clarkson, 38, and Blackstock, 43, filed for divorce on June 4, shocking fans because the couple had always stayed out of the tabloids.
The balance of their careers, caring for four kids and making time for each other became too much to juggle while the quarantined in Montana.
"It was a stressful time,” a source told People. “[Quarantining in Montana] exacerbated any issues.”
The “Kelly Clarkson Show” host and the music manager share daughter, River Rose, and son, Remington Alexander. Blackstock also has two children, Savannah and Seth, from a previous marriage.
“We've been in really close quarters and it's been kind of nuts. I'm not going to lie," Clarkson told People in May. "There's definitely some cabin fever going on. It's challenging being a working mom."
Another source claimed, "Things had changed in their marriage and the split was a while coming. They both work so hard. It's nearly impossible to enjoy each other, especially with the children needing constant attention."
Blackstock is the executive producer of the "Stronger" singer’s show on NBC and the two have a closely knit working relationship.
“Kelly and Brandon work together nonstop and that became a contention in their relationship,” an insider told Entertainment Tonight. "Brandon is the [executive producer] of her show, as well as her manager."
It is unclear whether Clarkson and Blackstock, whose stepmother is country music star Reba McEntire, will continue a working partnership after their divorce.