It's really easy to contribute negatively to a project. On the business end, I've worked with people who changed priority/direction/feature-sets so frequently that they actually made things worse; the project would have been better without them, hence a negative contribution.

On the technical side, have you never worked with a programmer that insisted on introducing his favorite framework/language, or declared that the "current code sucks, it needs to be re-written"? That's potentially a negative contribution. Hell I've even been that guy. It's not only possible, it's common.