Ask HN: Why do so many "Ask HN:" posts seem to cluster together?

I have noticed that quite often. Ask HN posts seem to clump together. Does anyone know if this is:

a. pure coincidence b. how HN ranks posts c. Some statistical correlation that exists between ask HN posts, that binds them more tightly than other posts. I would think it has to do with ask HN posts' objective: to be answered. d. total fluke just my experience