Is the anomaly caused by LW starting? Did it suddenly drop when we could post Discussion posts?

Yeah, that data was pretty inconclusive. Personally, I don't seem the point of open threads: Discussion allows you to talk about most things anyway, and it's way easier to check when someone makes a new Discussion post than a new top-level comment on an open thread.

Kudos for actually checking, though, and for publishing your inconclusive result.