A mechanism in which small twists can grow into vortices is investigated. A near vertical flux tube is modelled using 1.5-D MHD, with conditions matching those of the photosphere/convection zone. It is shown for a downflow that alters from being super-Alfvénic to sub-Alfvénic that an instability arises as a torsional Alfvénic twist propagates upwards against the flow. This instability occurs when the propagation of a series of Alfvénic twists stalls at the critical point (where v_z=c_A). Here, the small perturbations are amplified to form a non-linear twist with B_\theta>B_z. The Alfvénic twist forms into a shock as it decimates the pre-existing background flow. The growth of the Alfvénic twist may explain the motions seen in vortex observations such as Bonet et al. (2008;2010).