I think it refers to two things: the title of the book about how Linus create Linux “Just for Fun. The Story of an Accidental Revolutionary”, and the fact that Git had to be created to continue effective work on Linux kernel (after BitKeeper fiasco).

From the start of the article:

Linus Torvalds once wrote in a book that he created Linux just for fun, but it ended up sparking a revolution. Git, his second major creation, also an accidental revolution.