There exists strong experimental evidence that drastic changes of the geomagnetic field configuration take place in the near nightside magnetosphere before the onset of explosive phase of a substorm. A study of adiabatic effects of this field re-structuring in the particle distribution anisotropies is done in this work. It is shown that an externally induced non-stationarity of the magnetic field (most likely, due to increasing tail lobe magnetic flux) plays an important role in developing a field-aligned plasma pressure anisotropy, which is necessary for formation of an intense and thin current sheet at xGSM ∼ − 8RE. Other sources of the anisotropy related to the pitch angle dependence of the drift velocities are also discussed.