Hi Stu, Actually we do not yet know for sure the exact mechanism how coronal mass ejections (CMEs) form! To make things even more complicated not all CMEs are alike. They have different sizes and shapes and they are expelled from the Sun with large range of speeds. There are several theories how scientists think CMEs might be initiated. Many models assume that the energy is stored in the magnetic fields in the corona that suddenly becomes unstable and is released.
NASA launched a new mission called Solar Terrestrial Relation Observatory (STEREO) in 2006 that at the first time makes three dimensional images of the solar phenomena. Hopefully STEREO will help us to answer the puzzle of the CME initiation.