These accounts are curation bots. They try to get a good share of the curation rewards by automatically placing well-timed votes on posts and comments that are likely to get a certain payout value. Finding the right spot can give curation rewards worth much more than the value of the vote itself.
This happens not only on musing, but also on regular posts from established authors, on other Steem dApps like steemhunt and utopian, their moderator comments, and also on "known" self-voters or bid-bot users.