The origin of permafrost
What caused a centuries long freeze, the beginning of the current ice age. Yes we are in an ice age, as long as greenland and antarctica have large ice sheets we are technically in an ice age) Which in turn was caused by the movement of continents close enough to the poles to create permanent ice caps. the freezing of organic matter into permafrost is not a fast occurrence, it is a slow build up of organics as soil is built and added too, as long as it remains cold enough for permafrost to persist.  

The strange thing about the warming now is every major natural factor and most minor factors point towards cooling conditions yet we have warming. Human activity however is producing strong warming factors that are overriding the natural prevailing conditions, which makes you wonder what will happen when some of the natural factors stary cycling back to warming.