The currently favoured model to explain the development of the Younger Dryas stadial is developed by Wallace Broecker. He beleives that these changes in climate are triggered by a collapse of what he refers to as the 'Conveyor Belt'.
It shows how such such a shutdown could well produce the regional pattern of climatic impacts associated with the Younger Dryas. The rapid retreat of ice, driven originally by Milankovitch variations, gives way to warm, salty water which when it gets north, loses it's heat and sinks therefore starting up the conveyor. As the conveyor transported more and more heat to the north, it accelerated the retreat of the ice. (These Milankovitch cycles explain much longer periods of retreat and advance.)
Unfortunately, this model does not entirely explain the existance of Younger Dryas-like conditions in the northeastern Pacific or the rapid fluctuations found in Greenland. Other research has suggested that the terminations of the Younger Dryas cold phase were associated with Heinrich events.