There are a variety of reasons memes will feature as anonymous.
Reddit users who don't have an Imgflip account can use the Imgflip Meme Generator to make a creation and cross post that creation on both sites. I don't think user comments here on those creations even get to the Reddit user. When I see a single negative comment on a creation like that, I picture a person using a megaphone in an empty auditorium, their comment echoing off the wall.
Sometimes a user will submit a creation and then delete their account before that submission features. I think the user TammyFaye said goodbye to Imgflip that way.
Sometimes newer Imgflip users will "test the water" with an anonymous submission. I cringe when I think of the welcome wagon that shows up sometimes.