My theory is that it's because social media systems encourage and publicise agreement, while discouraging and hiding disagreement. This leads to a huge positive feedback effect on any meme that catches even a tiny number of people's attention, no matter how crazy.

On twitter, you signify agreement by retweeting; this takes seconds and permanently adds to the tweet's score. But to disagree, I have to write a message of my own. It'll show up in the conversation feed but doesn't affect the original message. And each disagreement stands alone, so they'll never accumulate a high score, so even the people who see them won't consider them as important as the original message.

Let's say somebody posts a Twitter message about, oh, how they've just tried cleaning their ear using trained caterpillars. It gets passed around a bit among the ear-canal-caterpillar-cleaning community, all five of them. But then a friend-of-a-friend-of-an-acquaintance retweets it, and suddenly thousands of people see it, and it's off-the-wall enough that a small percentage of them pass it on.

Suddenly it's got thousands of retweets. It's become a movement. Bloggers write about it, a few mainstream journalists comment on it, and abruptly Caterpillar-Facilitated Ear-canal Maintainance is a thing.

Meanwhile have written a tweet about how this is a really crazy idea and don't do it, seriously, it's just dumb. It'll get passed on a bit among my friends and will peter out.

It's not just Twitter; most social media networks do this.

Maybe we just need a 'dislike' button along with the 'like' button. It'd certainly be an interesting experiment.