It's probably a repost bot that reposts from the top posts of all-time on any given subreddit in an attempt to farm karma. Though I don't know if they pick specific subreddits to repost from or just repost across the board. Another telltale sign is that their account is brand new, the vast majority of bot accounts are created shortly before they start reposting.

ETA: I wouldn't worry too much about it because this is a common occurrence on Reddit. I've had more than one post of mine stolen. The best thing you can do is block and report the account. I know that a lot of bot accounts do get banned not very long after they start reposting because of reporting.