How about, SPHEW is inspiring bullies to declare themselves, or non-bullies to become bullies, for the challenge of battling SPHEW? (I'd like this hypothesis better if the bullies weren't older, since even acknowledging first-years as a challenge is quite a status hit.)
This seems like what is currently happening in the latest chapter.
even acknowledging first-years as a challenge is quite a status hit
Neatly sidestepped in Quirell's explanation.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: