It might have something to do with some gifs being embedded as videos due to the way that imgur processed them. Maybe that is the problem?
For example, I was able to do most gifs in this thread without issue, but this post had a gif embedded like this
[upl-image-preview url=//i.imgur.com/XTWJHCe.gif]</UPL-IMAGE-PREVIEW></p>
I had to download the image to my hard drive and reupload it as a video (?) file