Behind The Elevator Beatdown! Friends Say Solange Knowles Had Been Upset With Brother-In-Law Jay Z 'For Weeks' Because He Refused To Let Her Perform On His Upcoming Tour

Solange Knowles, had already been furious with Jay Z for weeks when she infamously attacked her brother-in-law in a hotel elevator after the Met Gala, RadarOnline.com can exclusively reveal.

Sources say the singer, 27, was livid that the music mogul, 44, wouldn’t allow her to perform on his and wife Beyonce’s upcoming “On The Run” tour.

"Solange had expected to be named the opening act for Beyonce and Jay,” an insider tells RadarOnline.com of the highly-anticipated tour, which kicks off in Miami on June 25. “She always travels with Beyonce when she tours. Even though she hasn't released new music since 2012, she thought it would just be a given.”

However, the rapper wasn’t exactly on board.

“Everyone knows that Solange was dumped from Jay's label for a reason,” the insider explains.

Now Beyonce is taking Solange’s side, and trying to convince her husband to give her struggling little sis the gig.

Drug Addiction, Steamy Affairs & A Hidden Love Child: Beyonce & Jay Z’s Top 30 Most Shocking Secrets & Scandals

“Beyonce wants to let Solange be their opening act just to appease her, even if it's only for a few shows,” the insider says. “It's unlikely this is what caused Solange to snap in the elevator, but she had been pissed off at Jay for a while. There is always drama with her."

As RadarOnline.com previously reported,cameras caught a violent Solange kicking and punching Jay Z in the elevator of New York’s Standard hotel during a Met Ball after-party.

This is just the latest erratic behavior from the DJ and singer, who abruptly cancelled her European tour last summer amidst rumors of drug use.