The origin of magnetism around AGB stars is uncertain. If these stars drive a dynamo, it cannot be sufficient to generate a strong global field, otherwise the observed X-ray luminosities would be higher. Other explanations for the circular polarization of SiO masers in the atmospheres are needed. The interaction of the AGB wind with previously ejected matter and with planets bears complex magneto-hydrodynamic phenomena on a short time scale, such that strong magnetic fields can be maintained locally. Here we provide observational evidence for the corresponding magnetic fluctuations. We trace magnetic activity with the circular polarization of the v=1, J=2-1 SiO masers, using a correlation polarimeter. In V Cam and R Leo, we find evidence of pseudo-periodic fluctuations of the circular polarization on a timescale of a few hours, from which we infer magnetic fluctuations of ~1G. The phenomenon is rare and restricted to a narrow range of velocities. It seems to be associated with planetary wake flows suggested by VLBI maps. While scenarios involving magnetic activity in the extended stellar atmosphere have problems to explain all observed features, precessing Jovian magnetospheres predict all of them. For the case of R Leo, we constrain the orbit of the planet (estimated period 5.2 years) and estimate a stellar mass of 0.7 solar masses.