Kelly Clarkson and Brandon Blackstock's split shocked fans and even their loved ones, but sources tell PEOPLE the couple have been facing issues for some time.
"It just came out of nowhere. Their marriage was so strong," says a source close to Blackstock and Clarkson, who filed for divorce on June 4.
But with busy careers and four kids, any cracks in the marriage became amplified while they self-isolated in Montana amid the coronavirus pandemic over the last couple of months.
"It was a stressful time," says a source. "[Quarantining in Montana] exacerbated any issues."
After they tied the knot in late 2013, the Voice coach, 38, and the music manager, 43, quickly welcomed daughter River Rose, 6. Two years later, their son, Remington Alexander, 4, joined the family, which included Blackstock's children, Savannah, 18, and Seth, 13, from his previous marriage.
"We've been in really close quarters and it's been kind of nuts. I'm not going to lie," Clarkson — who continued working on her upcoming album and filming remote episodes of The Voice and The Kelly Clarkson Show — told PEOPLE in May about life in Montana. "There's definitely some cabin fever going on. It's challenging being a working mom."
According to another insider, Clarkson and Blackstock have "been like friends for a while," and work demands put a strain on their relationship.
"Things had changed in their marriage and the split was a while coming," says the insider. "They both work so hard. It's nearly impossible to enjoy each other, especially with the children needing constant attention."
Now back in Los Angeles, the star is continuing to work and focusing on her children. "Kelly's been through a lot, and she thought Brandon was her happily ever after," says the source. "It's very sad."
For all the details on Kelly Clarkson and Brandon Blackstock's shocking split, pick up the latest issue of PEOPLE, on newsstands Friday.