It is usually the same motivations, typically one or more of the following:
1. Money - the most likely in vast majority of cases, even for someone like Biixi Imaan. Isaaqs can very quickly get a lot of under the table money and a good salary because the FGS is often desperate to have someone represent the Isaaq community and will pay them a lot of money up front. It is attractive even to someone like Biixi because he can command a higher figure than the average taxi driver they pluck of the streets of Xamar.
2. Power/position. You have many failed SL politicians seeing the road to a high government position being made easy if they betray SL and go south. Examples include Ina Cumar Carte Ghaalib & Guleed Ismaaciil. They failed miserably in SL politics so they take the easy, get rich quick road by switching to Somalia.
3. Qabiil motivations. You'll notice that a much higher proportion of Isaaqs in Xamar politics are GX, than other subs. Some in that community have lately felt (incorrectly) that SL is becoming a HJ/HA stronghold. If someone like Cirro or Cabdikariim Mooge (current Hargeisa mayor and fast riser in SL politics), gets to the presidency, this trend will reverse quickly.