It might happen that folks downvote things even if they like it, just because its the only interaction possible. From what little I understand of content reception, the drive to interact triumphs the drive to curate/(give your opinion for the collective good, for the lack of a better word).

In a few weeks the website would then introduce a ‘controversial’ or ‘worst’ sorting and ironic downvoting would become mainstream till the devs give in and treat the downvote as an upvote, or introduce both downvote and upvote (wherein imo they’d stand a chance of losing their userbase).