why some people are induced to change their life by it (perhaps only because it piques their interest for other material on LessWrong)

I'm also kind of surprised by this but... actually how rare really is it for people to say "X caused me to change my life."

I do know for a fact that people have changed their lives based on canon Harry Potter, with hundreds of people becoming obsessed with different character pairings etc. So maybe it isn't too surprising that it would happen with HPMoR, at least to a few people.

A weird fact about humanity and mass society is that virtually anything that reaches a large enough audience will wind up with some obsessive fans. As an example, dozens of women pledged their undying devotion to Richard Ramirez, the "Night Stalker."