They can't guarantee a random picture uploaded by a random user isn't porn. (though they give a best effort). Additionally the side bars show what's most popular. In theory something NSFW could end up with a thumbnail on an unrelated picture. So they give you the warning ahead of time. (I've used imgur tons and never seen this.)

This picture was a graph.

It did ask me if the image contained matured content, to which I said no. But for some reason it still marked it as mature? Very weird...