Reproducible major disruptions of low aspect ratio plasmas (R/a = 2.17) obtained in the Tokoloshe tokamak, have been studied. These disruptions are triggered by contact of a large m = 2 island with the inside of the limiter. The m = 2 growth is precipitated by a thermal collapse of the plasma periphery during the current decay phase. The time-scale of this island growth is strongly dependent on relatively small changes in the gas puff rate affecting the shear at the q = 2 surface. As a result, the start of the m = 2 growth before the disruption is not necessarily at all closely linked to the start of the thermal collapse. Clear evidence is found that the m = 1 mode plays no direct role in these major disruptions, nor does any other mode except the m = 2 appear to be important.