I've only been reading Aquaman since the Throne of Atlantis story line so maybe I'm missing something, but what was up with issue 20? That just seemed like a weird story that came out of nowhere to me.
I've also really been enjoying the Shazam backup story in Justice League. It will be interesting to see where they go with him after next issue.
In the trade its called a "fill-in". It either helps the regular team meet a deadline or lets them take a break.