Here's some info from Crowdstack Tech Support. I don't know if this applies here or not.
"...this is typically caused by users having the posting form open for too long and their session times out (more than an hour) before they submit it. When this happens on new posts, it would usually mean the attachments aren't associated with the post at all, but if they are editing the post, it makes sense that attachments would stay in place (as attachments), but might no longer be embedded..."
