I think a lot of it comes down to people visiting user's profiles. A known user like Raydog, ghostofchurch, or DashHopes will have people visiting their profiles, which other users don't have (people can't type in someone's name at the top of their browser if they don't even know what name to type).
Also, if you already have a meme on the front page, or are one of the top 10 scorers for the last week, it's really easy for people to click your username and go straight to your profile to see your other memes. So the way imgflip is set up, success on the site tends to get you more visits to your profile, and more visits to your profile are a factor in continuing success on the site. At least, that's my take on it.