There could be many ways. Maybe one person shared it on a large fanfiction platform and it gradually caught on. It might have been shared on social media by someone with a large following within the Harry Potter fan community.
Perhaps it started with a particularly controversial or well - written piece of fanfiction. If it was well - crafted in terms of the writing, it could have drawn the attention of readers who then spread the word. Another possibility is that there was some sort of event within the fan community that led to this fanfiction being highlighted, like a fanfiction competition or a discussion about dark themes in fanfiction.
The situation might have come about due to the interconnected nature of the fan community. Fans often communicate with each other through various channels such as forums, Discord servers, and Tumblr. If a piece of fanfiction about Harry being abused was posted on one of these platforms, it could have quickly spread. Also, if there are certain groups or individuals who are known for promoting unique or controversial fanfictions, they might have been involved in making 'everyone learns harry abused fanfiction' a known thing. Additionally, the popularity of the Harry Potter series means that any new and different take on the story, even a disturbing one like this, is likely to get noticed and talked about.