What prompted Solange’s alleged attack on Jay Z?

Solange Knowles’ alleged post-Met Gala attack on brother-in-law Jay Z sure surprised the public — but the tirade reportedly was an all-too-familiar scene for those closest to Beyoncé’s little sister.

“Solange has flipped her lid like that before,” a source told Us Weekly.

A video surfaced via TMZ.com on Monday of 27-year-old that allegedly showed Solange violently assaulting the rapper in an elevator at The Standard Hotel, where the trio attended a party following the Met Gala on May 6. In the footage, Solange is seen kicking, punching and screaming at Jay Z, before being restrained by an apparent bodyguard, while Beyoncé, 32, stands by watching the attack.

“This is not the first time this has happened,” the source said. “That’s why Bey and Jay were so calm. They are used to this from Solange.”

The source added that Solange is “genuinely a sweet and nice person but she also keeps [expletive] real.”

“She can take a small thing and it triggers her,” the source said.

The trigger that evening may have been a fight the singer-DJ reportedly had with designer pal Rachel Roy at the gala.

Meanwhile, other reports have blamed too much alcohol for Solange’s alleged assault on 44-year-old Jay Z.