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"It has come to my knowledge that a squib working for the Order of the Phoenix is attempting to gather an army of loyal muggles for their cause. She is writing novels about Potter's time at Hogwarts in an attempt to encourage muggles the world over to be aware of their immanent death or some such," laughed Lord Voldermort to his inner circle, "how comes you didn't pass this information on Severus."
"My Lord, I swear this is the first I hear..."
"Legilimens! Hmmm, Potter's potions class; Dumbledore moaning about DADA teachers, he should have given me the post when I applied; ah Order meetings" Snape could feel the Dark Lord searching through his memory as if it were a filing system and hoped that he had managed to hide the most important information from him. "You think such a plan is insanity and that had it been brought up at a meeting Dumbledore would have been laughed out of his own secret society," continued Lord Voldermort, "I have to agree with you the whole idea is preposterous. However it seems to be working. From what I have been told, the loyal muggle supporters are communicating with one another via a medium called the internet. They write coded messages via stories posted on fanfiction websites. We must infiltrate them. I believe their codes are biased around the romantic pairings that they have. Many of which involve the slander of having pureblooded members of the house of Slytherin copulating with blood traitors half bloods and mud bloods! This can continue no longer. Each one of you must write enough fanfiction to derail their plots. You must research the preferred plot lines for different character pairings and produce enough stories, which match them so that their codes are left unsuable. Now go, and happy writing."
And this is how Harry Potter fanfiction became so popular.