I believe those are usually on popular templates. There are two types of subimissions, one with imgflips approved templates, and those with a new picture that they haven't seen yet. If you use the approved or "popular" templates they have an autoflag feature for curse words or certain other words for whatever reason. If they don't have any of those words on them they autofeature without a mod having to look at them. If you submit a new pic however, a mod has to approve it to make sure it's not porn or something offensive. That human factor is what can make it take a long time.