Seems like when the boyfriend and the security guard were talking, the dog became protective of the boyfriend and broke the chain.  This would explain why the dog broke the chain and why the girlfriend couldn't hold him by the collar.  I've seen dogs act this way when I'm just talking to their owner.  They don't know that I'm not being a threat.  From what I see dogs are very gentle towards their owners and family members and very protective...but not to strangers.