Tokamak plasma current start-up assisted by Electron Bernstein waves (EBW) has been demonstrated successfully in a number of experiments. The dynamic start-up phase involves a change in field topology, as the initially open magnetic field lines form closed flux surfaces (CFS) under the initiation of a plasma current. This change in field topology will bring about a change in the current drive (CD) mechanism, and, although various mechanisms have been proposed to explain the formation of CFS, no detailed theoretical studies have previously been undertaken. Here, we report on the development of a start-up model for EBW-assisted plasma current start-up in MAST. It is shown that collisions are responsible for only a small part of the CD, while the open magnetic field line configuration leads to an asymmetric confinement of electrons, which is responsible for the greater part of the CD.