From the article's comments page, this theory is just typical of what I expected to come:
In the summer, Drudge was airing speculation that Romney might pick Petraeus as his running mate. This probably led the White House to have Holder order up some dirt on Petraeus, as insurance. So, that's why this investigation got started. Of course, Petraeus did not get picked as Romney's running-mate, so Obama no longer needed this dirt, which is why the investigation petered out. But then Cantor steps in, effectively telling the FBI "We know. You can't cover this up." And the rest is history. All started because of Drudge's rumor, and Obama's desire to obtain blackmail material on a potential opponent.