Details are scarce, as is unfortunately common in these kinds of matters, but we do know the most important fact: "the fight began because the victim did not know that singer Beyoncé's husband is rapper Jay-Z."
The fight also involved a music video, and likely also alcohol since it occurred at about 11 p.m. on New Year's Eve. Presumably the video depicted one or both performers, and this led to idle musings about the person or persons shown, a discussion of their various musical influences and respective relationships, and then some attempted murder. Exactly how that played out we may never know, although we can be sure it was stupid.
To be fair to the victim, according to People magazine the two superstars "always kept quiet about their relationship." They were married in a "really private affair" held in a big white tent on the roof of the building in which Jay-Z owns the penthouse, also keeping the event on the down low by having a bunch of giant silver candelabras and 60,000 orchids delivered the day before. Also, I guess it has been impossible for some time to not know, no matter how desperately one would prefer to not know, that a fetus is currently gestating inside Ms. Knowles at the behest of Mr. Z, soon to spring forth as a fully-formed celebrity after a birthing ceremony probably also to be held in a big white tent on Jay-Z's roof, to which for some reason Gwyneth Paltrow will also be invited. More on this and other celebrity doings in my new venture, Who Gives a Shit magazine, coming soon.
So I guess he really should have known. But I still wouldn't get too worked up about it.
The victim was reportedly in good condition at a local hospital, probably furiously paging through every magazine they have in order to make goddamn sure he knows who is married to whom, just in case it comes up again.