This is the Reddit announcement about their hosting [0]. At that point in time, Imgur was already redirecting direct image links to the non-direct wrapper page so that they could show banner ads. This made it difficult for Reddit to show images in the site and app well, thus the cooling of relations between both I had mentioned. It's interesting since Imgur started off as "an image hosting service that doesn't suck" according to the creator, on Reddit [1].
As I recall, it was Reddit who moved away from Imgur, not the reverse.