Unless something absolutely amazing is happening in the living room, the smokers will be out back, a pair of introverts will be in the hallway, and everyone else will slowly gravitate towards the kitchen until everyone is in the way.
It smells amazing too being near the action. Sight + smell getting stimulated all at once.
When the available seating is exhausted in the living room, it feels a bit strange to remain standing if everyone else is sitting, so you excuse yourself to the kitchen for another drink, where you find a number of other people standing and conversing. This feels less strange, so you don't see much point in going back to the living room.
Once everyone has made the inevitable trip to the kitchen to freshen up their beverage, and gets caught up in a conversation in the kitchen, the living room gradually becomes empty, and at that point, nobody wants to go sit alone in the living room and miss all the action happening in the kitchen.