What caused the Younger Dryas cold event? During the termination of the latest ice age the warming climate leading from glacial to interglacial conditions was abruptly reverted by a distinct but short (about 1300 years in duration) cold event, referred to as the Younger Dryas. This event resulted in substantial moraine zones around the southern edges of the Fennoscandian ice sheet, indicating a halt in retreat. There are also evidence of strong permafrost conditions in the periglacial area. What caused this 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/