It's interesting that (1) this thread appears in "Religion, Faith, and Theology > Abrahamic Religions > Islam," and (2) that at least one suggested explanation (Post #2) invoked a religious dynamic. An economic and political explanation may make more sense.
I always say: follow the money if you want to figure out who is behind activities that may be survivalist/mercenary action. In this case the terrorists appear to be funded by the rogue Eritrean regime. Eritrea is considered a rogue because they sought independence from Ethiopia in a manner that violated International Law. The Wiki gives you some idea of how Somalia became implicated in the Eritrean struggle for independence:
The fighting also spread to Somalia as both governments tried to out flank one another. The Eritrean government began supporting the Oromo Liberation Front, a rebel group seeking independence of Oromia from Ethiopia that was based in a part of Somalia controlled by Somali warlord Mohamed Farrah Aidid. Ethiopia retaliated by supporting groups in southern Somalia who were opposed to Aidid, and by renewing relations with the Islamic regime in Sudan – which is accused of supporting the Eritrean Islamic Salvation, a Sudan-based group that had launched attacks in the Eritrea-Sudan border region – while also lending support to various Eritrean rebel groups including a group known as the Eritrean Islamic Jihad.
Eritrean-Ethiopian War - Wikipedia, the free encyclopedia
To get a better sense for the situation, let's consider this: Somalia has been having an ongoing internal power struggle. Somalia's existing Western-backed government is actually run by Islamic insurgents. However, as a result of a split that occurred last year, there is now a competing faction that's apparently supported by militants from other countries.
Analysis: Somali infighting could help al-Qaida - Boston.com
Al-Shaab appears to be part of a larger network of factions who want to overthrow the existing Somali government. They are not Saudis. They are Somalis.
There is reason to believe Al-Shaab is getting outside support not only from the usual Paki and Saudi suspects, but also from the Eritrean regime.
American Chronicle | Eritrea Reportedly Arming AlShabab Militants.
My sense is that these regional conflicts and power struggles are very complicated - far more complicated than a single news article would lead you to believe.