According to New York Daily News, the argument that prompted Beyonce's sister to attack Jay-Z was all about the rapper's desire to go to a Met Gala after-party, hosted by Rihanna.
However it was Jay-Z's desire to attend alone, and let Beyonce go home without him, that angered Solange Knowles the most.
After telling his wife and sister he would be heading to Up & Down on W. 14th St., where Rihanna was holding her own post-Met Gala party, a source revealed that Solange turned to Jay-Z and snapped:
“Why can’t you go home?”
She then reportedly turned to Beyonce and asked: “Why does your husband need to go to the club right now?”
Jay-Z, who had apparently already rung ahead and told Rihanna's people he didn't need 'extra security' as Bey wasn't coming, supposedly took offence to Solange interfering in his business.
He apparently snapped: “You’re one to talk.”
His sharp reply reportedly incensed Solange, causing her to attack Jay Z with a series of furious kicks and punches in a three-minute outburst that was caught on surveillance video and leaked Monday.
The bad vibes between the pair had, however, started earlier in the evening.
A source revealed that Solange's friends had arrived at the Met Gala Ball and were 'pretending they were guests of his and not hers. Management went to Julius (Beyonce's bodyguard) and said there's a problem.'
Jay-Z, irritated, allegedly snapped at Solange, saying 'don't use my name'. This prompted her to say she wanted to leave the event.
A source explained to US Weekly: “Solange has flipped her lid like that before.
"She is genuinely a sweet and nice person but she also keeps s*** real. She can take a small thing and it triggers her.
They added: “This is not the first time this has happened.
“That’s why Bey and Jay were so calm. They are used to this from Solange. It has definitely happened before.”