While sources disagree on the details, the situation was apparently heated before Solange, Jay Z and Beyoncé stepped into the elevator.
According to one source, Solange had a run-in with designer Rachel Roy earlier in the night. "Solange was provoked by Rachel," the source says.
The source also says Beyoncé "intervened with a warning not to talk to her sister," adding that Jay Z did nothing to stop the alleged confrontation.
However, an onlooker has a different take, saying the women's interaction "was very limited" and that Solange was "yelling at other people at the party."
Roy has known the Knowles family and Jay Z for years. The designer, who is popular among celebrities, used to be married to Jay's former business partner and friend Damon Dash.
Then towards the end of the night, "Jay said something inappropriate to Beyoncé and Solange, and she snapped," says the source. "When they got in the elevator, it escalated quickly the way family tensions can. It got exceptionally heated the way family moments can."
Adds the source: "Solange is super protective of Beyoncé."
Was Alcohol a Factor?While the onlooker claims Solange was under the influence, the source says the singer "was not intoxicated" and has already moved on.
"She's fine now," the source says of Solange. "She's back home in New Orleans and she's going on with her business."
Reps for Solange, Jay Z and Roy did not respond to requests for comment.