It's a way to practice writing. Take a quick topic and see what you can make out of it.
Just like reddit's /r/WritingPrompts.
First, you don't have to be "into" Minesweeper to write fanfic (although maybe it helps). Second, since it was one of the games installed on every Windows computer for a few decades, it has an insane number of devoted players.
It's not 87 pages, it's 4 pages and 87 stories. I think it may be kind of a meta-fanfic-joke? Maybe? I Hope?