Actually it's the Kenyan military that has managed to dislodge Al Shabaab from significant swarths of Southern Somalia (up to their Mogadishu, and including the largest port, Kismayu). Ethiopia generally used to roll in, depose the warlord they didn't like, then head on back home. The Kenyan military tried to stabilize the region.

It's not all altruism though, the intention was to create a buffer near the Kenyan border as the country intends to build a large port in Lamu, a coastal town nearer Somalia than Mombasa, the current main port.

*Edit: Minor tense correction.