Maybe it's because the influx of new users are more casual in their use of the site, now that it has grown to a certain size. I could imagine that users that just stumble upon the site will be more willing to engage in this semi-destructible behaviour than users that got the site recommended from you or another early user.

Can you see whether the users that do this end up being good citizens? That would shed some light on whether these are malicious accounts or just new users trying things out.