It's war behind the scenes at Dancing With the Stars!

Though DWTS fans may have dismissed Maksim Chmerkovskiy and Derek Hough's testy public back-and-forth as simply a heated rivalry, the new issue of Us Weekly (out Friday) reveals what's really at the root of their bad blood.

"They hate each other," a DWTS insider tells Us of Chmerkovskiy, 31, and Hough, 26, who had it out after Chmerkovskiy's on-air spat with DWTS judge Len Goodman. (When the veteran judge slammed his and Hope Solo's routine, Chmerkovskiy lashed out and urged the crusty Brit to retire and called the ABC series "my show.")

"I had a laugh about that [comment]," three-time winner Hough told Us later. Referring to a nameplate on set that lists past champs (Chmerkovskiy is not one of them), Hough said, "I was looking at the plaque upstairs and…well, never mind."

Hough's recent snide retort is just the latest in the pros' lengthy feud.

"The professional dance community is really small. The dancers have known one another their entire lives," says a source close to the show. "Now that the group is famous, it's added a whole other level of rivalry and competitiveness."

Adding fuel to the fire between the men? Hough's sister, fellow dancer Julianne, who briefly dated Chmerkovskiy.

"Julianne told Maks she didn't want to have a serious relationship," says a source close to the show. Soon after, the Footloose star started seeing Ryan Seacrest -- and a bitter Chmerkovskiy, says the source, retaliated by "planting false rumors that Julianne was cheating on Ryan with Maks."

The DWTS insider counters that Chmerkovskiy is no rumormonger and, in fact, Hough isn't the nice guy he seems.

"Derek is dating Lauren Conrad for publicity," says the insider of his romance with the Hills star. "He is focused on becoming a huge star like Julianne."

For more on the pros' fiery feud -- including how it has divided the rest of the DWTS cast -- pick up the new issue of Us Weekly, out Friday!