TMZ has learned they felt like this was the right time to fly the coop since they have 2 kids now. When they moved in with mom, Kim was only pregnant with North.
As for their new digs ... you'll recall ... the couple bought the Bel-Air spread for $11 million in 2013 and immediately overhauled it. We're told renovations were completed a few months ago, and a moving truck pulled in Tuesday. But it's a temporary move, since the mansion's already on the market for $20 million.
Ultimately, Kimye will set up shop in a Hidden Hills estate they're building near Kris ... once that construction's complete in a couple months.