Illustration of four source-intrinsic phenomena that may occur in the inner part of an AGN and induce the astrometric position variations observed with VLBI. The scale used is not realistic. Panel A: ejection and motion of radio knots away from the VLBI core. Panel B: precessing jet. Panel C: VLBI core instabilities. Panel D: dual VLBI core and orbital motion due to the presence of a supermassive binary black hole.