Has it happened that two people with opposing philosophical views destroyed each others' reputations on Stack Exchange, after a voting war? I mean, one person could have cosmological presuppositions, which form his/her opinions. Another person could have ontological presuppositions, forming his/her opinions. Due to the logically different views, if a cosmological person down votes an ontological person's opinion an ontological person can then decide, well, s/he will also down vote the cosmological person's opinion; because doing to others as they do to selves is a universal law of social contract theory.
In theory, at least, answers should be judged on formal, (largely) objective criteria: Are they well-formed and clearly expressed? Do they directly address the question asked? Do they cite reputable sources?
In theory, questions that are not susceptible to answers of this type should be closed.
In theory, therefore, one's personal agreement or disagreement with the actual positions described in the answers should not affect your voting. In theory, we are neither expressing nor voting our own positions here, but rather providing information about the field and practice of philosophy, and voting how well others have supported that goal.
An upvote is 10 points, a downvote is -2. So if people upvote you and then others downvote you, you still gain points. So the overall reputation score does reflect the odds someone thinks your perspective adds value -- even if others do not -- rather than the balance of votes on your actual posts.
These two 'competitors' could easily both gain from a very contentious exchange that leaves most of the answers with scores of zero. If everyone on one side upvotes and those on the other downvote, the overall gain for the poster is still substantial.
Yes, this happens. But if there are 'leaders' of these contingents it often means that such people are violating the automated 'serial-downvoting rules'. If you are focussed on challenging a specific individual, and are not equally negative toward many others at the same time, a pattern will appear in your votes that will get you punished by having all of those votes against that individual stripped and discarded. So a set of leaders of a contingent that always disagrees with author X, may get their downvotes annulled.
I am not sure if this has happened on Philosophy SE, but I have received down votes on politics SE because people did not like my political views. I have not returned the favor and turned it into a voting war though.
The downvote machine rolls heavily on MathStackExchange and MathOverflow when a critic of set theory is detected. On the other hand you get upvoted when you support it. I would not exclude that meanwhile it happens also here by some hard-core adherents of actual infinity but of course I cannot identify them.