An unplanned stop for the night may have prompted Solange Knowles’ elevator attack on Jay Z at a Met Gala after-party at the Standard Hotel, reports the New York Daily News.

Beyoncé’s sister reportedly became upset with her brother-in-law after he announced his plans to leave the Standard Hotel event and head to Rihanna’s Met Gala after-party. The Daily News adds that ahead of the altercation in the elevator, the 27-year-old Knowles first became angry when a couple of her friends were unable to enter the party at the Standard.

“Two of her friends, who were not dressed for the Met Ball, showed up at The Standard and caused a scene downstairs,” the source told the Daily News. "They wouldn't leave and kept name dropping Jay Z. They were pretending they were guests of his and not hers. Management went to Julius (Beyoncé’s bodyguard) and said there's a problem."

According to one source, Jay Z learned that Solange’s friends were attempting to use his name to gain entry to the party. He became fed up with Solange, ordering her not to use his name. At that point, she reportedly became annoyed and was ready to leave the party.

As the trio prepared to leave the Standard Hotel, Jay Z said he would be going solo to a separate Met Ball after-party being held by Rihanna at the nearby club Up & Down, a second source said. His team had called the venue ahead letting them know he would be coming by himself and no extra security would be needed, the source continued.

This reportedly upset Solange, who then questioned why the rapper was staying out.

"She seemed drunk and irritated," a source said. "She said, ‘Why can't you go home?' and to Beyoncé, 'Why does your husband need to go to the club right now?'"

“You're one to talk," Jay-Z replied, the source said, setting off Solange’s violent outburst in the elevator. As the surveillance video obtained by TMZ shows, once Solange entered the elevator, she began kicking and punching her brother-in-law as Beyoncé looked on and her bodyguard attempted to restrain Solange.