A new stability analysis methodology is being developed at the Paul Scherrer Institute (PSI) using the best-estimate coupled neutronic/thermal- hydraulics code, SIMULATE-3K (S3K). This methodology has so far been validated against Leibstadt NPP (KKL) stability tests of C10, C13 and C19, which all show global (in-phase) oscillations. However, the methodology has not yet been validated for regional instabilities and to that aim, a special KKL cycle 07 stability test was selected. Indeed, during this test, the core not only showed growing power oscillation amplitudes in an out-of-phase regime but also an oscillating and rotating symmetry line. Thereby, it was selected in order to verify the S3K capability to predict regional instabilities and on that basis, obtain more insights towards understanding the causes for the oscillatory and rotational behaviour of symmetry lines. The results obtained so far are presented in this paper. First, it is found that the S3K results are in good agreement with measurements both qualitatively and quantitatively, although the resonance frequency is slightly over-predicted. Secondly, the excitation of the out-of-phase mode with oscillations as well as rotation of the symmetry line is also well captured i.e. in accordance to the experimental observations. Related to this, an in-depth analysis of LPRM signals indicates that two out-of-phase oscillation modes associated to two azimuthal neutronic modes are simultaneously excited. Furthermore, it is found that a superposition of these two modes will trigger the symmetry line dynamics and that the behaviour will be guided by the dominance ratio between these two modes. More precisely, the oscillatory behaviour is due to the superposition of the two azimuthal modes but with one dominant mode. The rotational behaviour is however due to the superposition of the two modes with comparable strengths.