The tricky business of dating medieval documents seems to have prompted a German historian named Heribert Illig to throw his hands up in the air and deny a whole chunk of the early Middle Ages existed. Neither did Charlemagne, who was a pretty big deal in the time 614-911, so he is relegated to a historical forgery along with all the various artifacts dated from that time.
The theory has more trouble with the rise of Islam, which occurred during the 'phantom' time and was something that definitely happened, as were the various eclipses and comet sightings that can be neatly lined up with the un-phantomed chronology. Not only European, but Islamic and Chinese history would need massive revisions to make the fraud work.
Why anyone would want to falsify a couple of centuries of history is as big a mystery as why there are people who want to believe it. Even assuming anyone was capable of coordinating a massive historical con job across the entire world, there's no reason for them to do so.
The only person with any motivation is Heribert Illig, whose desire to sell a book about his theory is probably the real driving force behind all this.