Until the advent of the VLBA most knowledge concerning the time variation of SiO masers and other tracers of circumstellar envelopes came from single dish and optical observations. Frequent VLBA observations enable us to produce a time-lapse movie of the v=1, J=1-0 43 GHz SiO masers in the inner circumstellar envelope of the isolated Mira variable TX Cam. Our observations currently cover almost a full phase of the stellar pulsation cycle and reveal several surprising facts. They show that, in contrast to previous observations and theoretical models, the gas in which the masers are embedded is undergoing expansion during the portion of the cycle when infall is predicted. The mean expansion velocity is ~3.5 km/s. Some isolated features are seen to start falling back towards the star at around stellar maximum and then, starting around phase ~0.2 highly complex motions are visible. I will attempt to explain the departure from expected behaviour by invoking the magnetic fields we have previously detected at the positions of the SiO masers.