Could the "resigned" military head and minster of defence have something to do with it?
They were probably pissed off at being forced to resign last week (We ain't like the Japanese that commit seppuku for the tiniest mistake ...they wanted to stay on).
They also still have huge influence over the security infrastructure in Mogadishu
... as well have access to explosive caches confiscated over a long time.
Don't want to speculate on their possible motivations or end goals but I can't seem to think of anyone else apart from them, Al Shabaab or a even scarier foreign trained group that could pull this off.
We could probably get more clues by seeing if any foreign countries declare new military initiatives over the next couple of weeks.