We recently had somebody who is not following our proposal (at least I'm pretty sure they are not) indiscriminately go through and downvote every question that has one or more votes without leaving any comments.
Apparently (see: How many downvotes do users get?) there is no limit to the number of questions you can downvote and no penalty for downvoting. This is a good mechanism to have in place in case people are spamming your proposal with bad/off-topic questions, but in this case it seems purely malicious.
Here are some ideas to prevent this:
Require some minimum level of reputation to downvote questions in a proposal that you are not following.
After each downvote, popup a comment field for the downvoter to provide feedback (maybe it should be a requirement to fill out the field, but even if it's optional this will still help). This is good practice anyway.