Somalia is a complex place with a long history of civil unrest fueled by clan dynamics that few us of westerners truly understand. Al Shabaab has made a name of itself by successfully sustaining operations and acquiring funds despite immense military pressure and elimination of leaders in drone strikes. The group’s funding stream has been a topic of intrigue for some time; today news outlets now claim Al Shabaab is funded in part by profits from wildlife poaching. For more information read Africa’s White Gold of Jihad: al Shabaab and Conflict Ivory by the Elephant Action League.
Within the last year the UN exposed links to the charcoal trade and documented an elaborate taxation structure in place at the Kismayo port. Analysts Compass analysis of US cases involving financiers provides insight into the mechanics of the group from a much different angle , much closer to home. There is no doubt that financing for the Nairobi attack will be a topic of great interest to intelligence agencies around the world.
This Washington Post article provides speculative insight into the reason for the attack.
For background information on Al Shabaab check out this PBS Rundown post