It's because as a low post user, they are still in the trial period. If they post a URL, it makes all the other posts moderation required. This is an anti-spam device. Summary: If you post links in your first 15 posts, it places them in moderation que to fight linkspam.
I have changed the settings to a more reasonable level. New users will still be moderated for first post url content.
What I've found said the problem is either another mod/ admin deleting posts or the site has been hacked. Possibly a server glitch or some sort of error in the web pipeline? Don't know of anything else. I've noticed this too.