New free image hosting services pop up every few months to try to take imgur down as the king of Reddit. So far, they've been unable to maintain any leads gained, but if imgur screws with Reddit they will be replaced in a heartbeat.
To keep traffic "on premises", ie thwart competition. Just look at imgur: now it's sort of a micro reddit, with its own commenting system, categories, and so on.
So you're a site with a major feature that (a) drives publishers to another site, and (b) drives viewers to the same site, even though (c) the content was created for your site. And now (d) that site is adding some of your core features. I guess I can't see how that isn't a blindingly obvious concern if you're running reddit.