It’s a wacky idea, and I don’t quite get it myself – read here for a long article explaining it. In part it says:

Mike Baillie …is saying that the Black Death, one of the most deadly pandemics in human history, said to have killed possibly two thirds of the entire population of Europe, not to mention millions all over the planet, probably wasn’t Bubonic Plague but was rather Death By Comet(s)!

…In an average year there is one atmospheric explosion with a yield of 100 kilotons or more. The large majority occur in such remote areas, or so high in the atmosphere, that they are not observed. Even if observed, the witnesses may see only a flash of light in the distance, or hear the ‘rumble of distant thunder’ coming from the open oceans. Thus even those that are observed are often not recognized.

What the article does highlight is a number of events in the past that have been attributed to dragons that may have Tunguska type events. It even suggests that such events occur roughly once a year (usually higher in the atmosphere than Tunguska), and one day it will hit a populated area unexpectedly.

It also points out that the Black Death begain 666 years prior to 2012, although according to Wikipedia it began a year later than that.