When it comes to fans of certain things, there are many people who are sane, down-to-earth individuals; individuals who understand that if they talk about a subject and the other person isn't interested, they'll change the subject. These people know how to keep their hobbies under control; how to differentiate between reality and their interests.
Unfortunately, these people aren't news. That's because Weird = Interesting; like any program on television, the news has to be interesting or else people won't watch it. Why talk to an individual who works in an office and occasionally picks up a copy of Spider-Man when you can talk to the 300-pound guy who spent $7000 on his exact replica of a Scarecrow costume?
The problem is that because only the weirdos get interviewed, this causes non-fans to believe they're all like that; for example, that all comic fans are clones of the Comic Book Guy from The Simpsons. This is also the main reason why people would hate something just because of its fanbase.
This phenomenon is not just limited to Comic Books; it extends to other hobbies, various sports, a lot of media, some professions, and even certain beliefs and philosophies that people hold, not to mention loads of stereotypes. Although there can be odd exceptions: compare how your average sports-fan treats a guy with his face painted in team colors versus a guy in a Spock costume.
Sadly, this concept serves very well for Chewbacca Defense. Someone complains about you doing something wrong? Stigmatize them as a Vocal Minority to prove that they are the one who's wrong, make it look like everyone agrees with you, and that complainers are just deluding themselves into thinking otherwise. That's Argumentum ad hominem, but what's more important is that history tends to avert this entirely; i.e. the mere fact that someone complains really hard means something is going very, very wrong. The poem “First they came…” by Martin Niemöller is all about supporting this point of view. It’s about Nazis. Specifically, it's about how groups unhappy with Hitler's rise to power (like communists or trade unionists) were considered a Vocal Minority, so no one really cared about their persecution and imprisonment. We all know what happens next.
Due to the nature of this trope, many fandom-related tropes are induced solely by this. The only members of a fandom who vocalize their opinion are the ones who feel so strongly about their opinion that they feel the need to vocalize it. The Scrappy, the Ensemble Darkhorse, the Fan Dumb, the Hate Dumb, and Fan-Preferred Couple are all based on what is commented on by the Vocal Minority. So, characters like Scrappy Doo and films like Transformers: Revenge of the Fallen both sound like the spawn of Satan when described by people on the internet, but usually have a much more positive description via Word of Mouth. In other words, asking someone on the internet about the two will often get long-winded, spiteful rants, while asking a guy on the street will often get much more praise. So, Scrappy Doo and Wesley Crusher could've very well been fairly well-liked by the silent majority. This is a major reason why Pandering to the Base can be so problematic; even if the other pitfalls are discounted, the writers may only be serving a small but vocal minority.
Still, a creator can sometimes benefit from paying attention to vocal minorities. Just because they're a minority, doesn't mean they're a small group: in large fandoms, the minority could number in the thousands. While blindly kowtowing to any fan's protests or demands is often a stupid and harmful idea, there are plenty of times where fans may actually be onto something, even if it isn't phrased in the most polite or reasonable manner.
This trope can occur in multiplayer internet games of nearly all types. One example would be an MMORPG in which most people can see that a given class or ability is overpowered compared to others, sometimes to the degree of being a Game-Breaker. If the creators try to remedy the imbalance, the most dedicated players (many of whom have been using the imbalance) may dominate online fora and insist that the changes are highly unfair. On occasion, the players will be polled on the topic, and if that happens, it's not unusual for those who object to be found to be no more than a Vocal Minority.