I don't have any proof but, I am pretty sure that the downvotes are given by a single person. Whoever has done this is, trying to game with the system. The only reason is that (s)he wants to provoke the auto-deletion.
You not only don't have any proof, you don't understand how auto-deletion works. Fortunately, I have a reference for this:
If the question is more than 30
days old, and ...
- has −1 or lower score
- has no answers
- is not locked
...or...
- it was closed and migrated to a different site
... it will be automatically deleted.
If the question is more than 365
days old, and ...
- has a score of 0 or a score of 1 with a deleted owner
- has no answers
- is not locked
- has a viewcount <= the age of the question in days times 1.5
- has 1 or 0 comments
... it will be automatically deleted.
These checks are run every week across all sites.
If the question was closed more than 9
days ago, and ...
- not closed as a duplicate
- has a score of 0 or less
- is not locked
- has no answers with a score > 0
- has no accepted answer
- has no pending reopen votes
- has not been edited in the past
9
days
... it will be automatically deleted
As you can clearly see, the question you're referencing doesn't qualify for auto-deletion. Furthermore, even if someone tried to make it qualify by downvoting all of the answers, you could single-handedly thwart their efforts by upvoting them. Of course, you should only do this if you thought the answers were useful...
...and if you thought the answers were useful, you should probably try to salvage the question as well. Notice that it's currently closed? That means it is eligible for deletion. Whenever you see a closed question, you should think to yourself "this question may be deleted at any time for the reason stated below it" - that's essentially what it means. Even if it doesn't qualify for auto-deletion, there's nothing stopping trusted members of the site from just deleting it manually - which they can do regardless of answer score as long as it is closed.
So while you're here hand-wringing over a supposed voter who you cannot prove exist, the fact remains that this question and its answers may be removed at any time regardless of the existence of your voter.
...Unless you do something about it. You could...
Edit the question to fix any issues with it. This may be laborious, but the option is available to anyone reading it. If you or anyone else sees value in that question, then you should fix the problems stated below it.
Vote to reopen the question. You haven't earned the privilege to do this yet, but nothing's stopping you from doing so. If you really care about this questions or others like it, earning the privilege to reopen would be a much better use of your time than throwing out unfounded accusations here.
Post a compelling justification for reopening here. More than enough people read this meta site to easily reopen any question they feel is worth keeping around; all you need to do is convince them. Conspiracy theories tend to not be all that persuasive, but if there's value in those answers then you should be able to explain why the question should be preserved. Do it calmly and clearly; you only need to convince 5 privileged users.