The explanation didn't sound right in the first place and, in the end, may not have existed at all.
Two months ago Ramtane Lamamra, the African Union's (AU) Commissioner for Peace and Security, announced that several Ethiopian battalions would redeploy out of Somalia by the end of April. The announcement left many questions unanswered and defied Ethiopia's recent gains against al-Shabaab, including February's capture of Beledweyne and Baidoa. Having opened an enormous front involving hundreds of trucks, armored personnel carriers and tanks, Addis Ababa would risk its advantages by transferring near-term control to green AU reinforcements and Somalia's Transitional Federal Government (TFG). Its pincer assault was so immediately successful that many Somalis welcomed the arrival of their historic enemy, and some demand that they stay for the foreseeable future.
Lamamra, himself an Algerian diplomat, did qualify his statements by predicting that Ethiopian troops will remain in central Somalia if they can be "re-hatted" like Kenya's, making them eligible for AU funding. This clue tipped off the probable reality that Addis Ababa never intended to withdraw, only secure international funding and deceive al-Shabaab's leadership, and its troops are headed for the same destination as Kenyan troops. On Saturday an Ethiopian Major General involved in the Somali campaign announced, "Truthfully Ethiopia has fulfilled its mission in Somalia, and it is not a problem for our military to liberate the port city of Kismayo." Prime Minister Meles Zenawi publicly ruled out a withdrawal soon afterward, congratulating Ethiopian troops and promising to "take over more key strongholds."
Determining the exact cause behind Ethiopia's actions from the outside is impossible, but its motivation combines a number of factors. If the government wasn't employing deception from the start, military officials on the receiving end of positive local feedback may have convinced their political superiors to keep advancing into Somalia. Both parties would have experienced second thoughts about leaving a vacuum along Ethiopia's border and straining the TFG's capacity.
Another distinct factor is the slow pace of Kenya's Operation Linda Nchi, which has yet to reach Kismayo since its launch in October 2011. The TFG and AU's ideal strategy planned to secure all major population centers before August's presidential election, an event that will theoretically end the UN's current roadmap and established a more representative government. Kenyan forces were supposed to besiege Kismayo by February, according to Ethiopia's Major General John, but they remain locked in peacekeeping operations between the port and their border. Some observers attribute Kenya's pace to poor military planning, inexperience or a cautious approach, in contrast to Ethiopia's blitzkrieg movements. Garowe Online tackled this issue directly, reporting that Kenya ”has made little progress against Al Shabaab compared to Ethiopian troops.”
The likeliest explanation fuses all factors together.
As is the case with general war strategy, summoning Ethiopian troops to finish the job generates a matrix of risk and reward. The TFG-AU conceived a three-prong battle plan to restore an image of national stability to Somalia, deploying an influx of AU reinforcements to Mogadishu as Ethiopian and Kenyan troops advance from the west and south. The three forces would expand their area of operations until they overlap, then gradually turn local authority over to the TFG, but the plan appears to be taking longer than anticipated. Armored and ready for new battles, Ethiopian troops could be the only force capable of dislodging al-Shabaab from Kismayo before August and speeding up the AU's designs. By seizing all of the insurgency's strongholds (Beledweyne, Baidoa, Bardahaarrey, Baardehere, Kismayo, Afgoye), the AU hopes to flush al-Shabaab out of its remaining water, sever its economic network and dismantle the group into smashable pieces.
Clearing space for August's election is also vital to Somalia's stability; al-Shabaab has already interfered with clan leaders headed for Mogadishu and is expected to disrupt the election on the politico-security front. [Update: due to security concerns, the UN has decided to elect Somalia's next president through a centralized parliamentary vote in Mogadishu.]
However racing to Kismayo may not be possible before July. Deep as Ethiopia has treaded into Somalia, a 200-mile journey and multiple river crossings separates Baidoa from al-Shabaab's southern port. This distance will stretch Ethiopia's supply lines far from its border, making them more susceptible to ambushes and IEDs, and remove Ethiopian troops from an established comfort zone. The army did travel towards Kismayo in 2007, forcing tribal elders to evict the Islamic Courts Union (al-Shabaab's predecessor), but a formal incursion is likely to trigger more resistance. 2007’s eventual outcome must also be avoided: Somali militants have been forced out of Mogadishu and regional centers numerous times, only to re-infiltrate them when no government entity stood up on two legs. Although Ethiopia presumably has a plan to clear Kismayo, its plan to govern is likely no more advanced than Kenya's.
Capturing individual cities, even a string of them, represents a specific type of mission. Holding all of Somalia's south-central territory presents its own challenge, one that time may force the AU and TFG to break up into pieces.