This must mean that someone saw the title of it at -11 karma and thought "Sounds promising! Reading this now will be a good use of my time." And then they read it and went: "Arrgh! This turned out to be a disappointing post. Less like this, please. I'd better downvote it to warn others."

Not necessarily. Seeing a heavily downvoted post seems to trigger some kind of group-norm-reinforcement instinct in me: I often end up wanting to read it in the hopes of it being just as bad as the downvotes imply, so that I could join in the others in downvoting it. And I actually get pleasure out of being able to downvote it.

I'm not very proud of acting on that impulse, especially since I'm not going to be able to objectively evaluate a post's merit if I start reading it while hoping it to be bad. But sometimes I do act on it regardless. (I didn't do that with your post, though.)