The interaction of tip vortices and the tower of a model horizontal axis wind turbine is investigated by means of high-fidelity detached-delayed eddy simulations. The objective is to understand the interaction of tip vortices with the tower and how these cause instabilities to grow. To get a first overview of the general flow field, the time-averaged axial velocity is evaluated in different downstream locations. These indicate that partial tip vortex breakdown starts downstream the tower as there is no sharp shear layer visible separating the wake from the outside flow. These areas grow in circumferential direction the farther the fluid travels downstream. The analysis of the tip vortices indicates that these instabilities are caused by a periodically changing vorticity and axial velocity component in the vortex centre. Additionally, the vortex gets deformed into an elliptic shape and the vortex centre is displaced in the order of the vortex core radius. These instabilities are propagated along the tip vortex trajectory and lead ultimately to an accelerated tip vortex breakdown.