What caused the Younger Dryas cold event?
The most accepted theory is that the Younger Dryas was caused by a large reduction or shutdown of the North Atlantic "Conveyor" because of a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. 
(Although evidence for such an event is thus far lacking.) The global climate would then have become locked into this new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. 

Another theory suggests that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, which then brought more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation.

http://geosciencesocietysio.wordpress.com/2014/05/04/paleoclimate-what-caused-the-younger-dryas-event/