I upvoted the post because it had negative karma, and was not a post that I thought should be at negative karma.

In general I vote posts/comments in the direction I think their karma should be at. Thus for instance I downvoted Clippy's comment above because I did not think it was so insightful that it merited 20+ karma. I would not have downvoted it if it were at 0 karma.

I assume many people take this approach (it fits in nicely with consequentialism) so this probably explains what you saw.