Also, consider that the process of organic material decaying once buried can take quite a lot of time, meaning that burial rates don't have to be as rapid as you might think. Modern-day sphagnum peat bogs accumulate a lot of organic material in them even in temperate climates.
What process caused such a sudden, centuries-long freeze? Don't we need to know this before we understand why it's reversing now? Presumably we humans had nothing to do with the freeze!
As stated above, the drop in temperatures doesn't have to be rapid because organic material can accumulate in permafrost even in a stable climate.