There has been some excellent analysis online of the Al Shabaab attack in Uganda. Ultimately, there are two common motivations offered:
First, Shabaab had threatened both Uganda and Burundi over their provision of troop to the African Union Mission in Somalia (AMISOM), which is supporting the Transitional Federal Government in Somalia — the weak authority put [...]

AFP: Uganda Bombings Kill 74 at World Cup Parties
Last night, two explosions went off in two restaurants in the Ugandan capital, killing at least 74 people amongst crowds gathering to watch the World Cup final. Though no group has yet claimed responsibility, initial reports suggest that Somalia’s al Shabaab, which espouses a harsh interpretation of Sharia [...]

The Washington Post ran this story a few days ago and I wanted to belatedly say a few words:
In Somalia’s war, a new challenger is pushing back radical al-Shabab militia
The militia, a Sufi group, is posing the strongest challenge yet to al-Shabab, an al-Qaeda-linked organization. The Sufis potentially offer an alternative strategy for the United [...]

From the Washington Examiner:
Al Qaeda Reaps Recruits from Somali Refugees in Yemen
Al Qaeda’s robust terror organization in Yemen in recuriting from a pool of hundreds of thousands of Somali refugees who have fled war in their homeland
If AQAP is able to successfully incorporate Somali refugees into its organization, its options for conducting attacks abroad could [...]

Somalis with terrorist links feared headed to U.S. border – Washington Examiner
Somalis with ties to a terrorist organization are believed to be plotting to illegally enter the United States after being mistakenly released from custody in Mexico.
All were in Mexican Immigration custody due to illegal entry into Mexico and released on January 21, 2010.
It’s difficult [...]

Mogadishu Residents Told to Leave Somali Capital – BBC
Mogadishu’s mayor has told residents to leave the Somali capital’s war zones, amid fierce battles with insurgents. Mayor Abdurisaq Mohamed Nor said the long-anticipated government offensive may start soon, so residents should withdraw at least 2km (1.25 miles)
It is questionable whether the US-backed Transitional Federal Government’s [...]

Brought to U.S., Man Is Charged With Aiding Somali Terrorists – New York Times
This past Monday, Nigerian authorities sent Mohamed Ibrahim Ahmed to the United States to face charges that he provided material support to al-Shabaab, the terrorist insurgent group fighting to overthrow the Transitional Federal Government (TFG) in Somalia. Ahmed is alleged to have [...]