As they held each other at Katherine Heigl's birthday party, no one would have thought anything was wrong between Kate Walsh and her husband of 14 months.

"They were dancing up a storm, having a really good time together," a Star magazine source says of Kate Walsh and Alex Young, a movie studio executive.

"There were no signs that there was any trouble in paradise."

Yet just a week after Katherine's November 15 bash at her mother's L.A. house, Kate and hubby Alex Young called it quits, according to divorce papers.

So what went wrong in such a short time between the Private Practice cast member and her husband? Friends say one of the main reasons her marriage has gone south has been Kate's growing closeness with David Sutcliffe.

That's the divorced hunk who plays her love interest on Private Practice. As Kate's Dr. Addison Montgomery and David's character, Kevin Nelson, have grown closer since he turned up at Oceanside Wellness, so have the actors.

"Kate really likes David, and knowing that didn't help Alex's fragile ego," a friend of the actress says. "Kate and David had a real connection playing lovers on the show, and she didn't seem to have that with her own husband."

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.