Why did my commit get a -2?

A -2 is the opposite of a +2, and means "do not merge". It can be seen as a stronger -1. -2's are sticky, and will remain on subsequent patches until removed.

-2's are used for multiple reasons, sometimes by the author themselves to indicate a certain patch is a work in progress and should not be merged. Other times a patch may be conceptually flawed and will be never merged. When a controversial patch is -2'd, it often results in further discussion, which leads to an amicable solution where the original changeset is merged.