According to reports from the New York Daily News, Jay Z donned his shield and did what he had to do to protect his wife from being injured in the fight that broke out during the Dream Downtown VMA after-party.  An alleged guest at the Dream reported to Confidenti@l:

"Diddy got in a fight with someone partying too close to him and his girlfriend.  It was mayhem. Beyoncé was yelling to stop and Jay Z was separating people and getting people away from Bey.  We were all dancing and the fight breaks out, and it happened so quickly and bottles were crashing everywhere, and everyone was screaming. People got pushed into bottles.  Then Diddy got up, took the mic and said 'It's all cool! Ya'll keep having fun! It's no big deal, it's over.'" 

The fight, which began around 1:30 a.m., apparently lasted for approximately 90 seconds.  

Busta Rhymes allegedly entered the scuffle with a bodyguard, most likely to break it up, as Jamie Foxx, Pharrell, Will.i.am, and Russell Simmons, among others, looked on at the mayhem.

Source: bossip.com