Al-Shabbab is saying that it is a military move, but whatever it is, it is probably going to save some lives:

The Shabab Islamist rebel group abruptly pulled out of the bullet-ridden capital of Somalia on Saturday, leaving the entire city in the hands of the government for the first time in years and raising hopes that aid groups could now deliver aid to more famine victims unfettered.

[...]

The Shabab's notoriously brutal brand of Islamism had tormented Mogadishu residents for years, but more recently the rebels have also blocked many international relief groups from bringing food to victims of the famine that swept southern Somalia this summer.

Aid groups hope that the Shabab retreat will allow them access to more parts of Mogadishu, where more than 100,000 famine victims are waiting for help. But the Shabab still controls large parts of southern malia that have been worst hit by drought and famine, and it is not clear when Western aid groups will be able to get into these areas.

African Union troops joined government soldiers in moving into Mogadishu which is a good sign. The AU hasn't been very active in assisting the government and it could be that this famine has galavinized them into action.