In conventional metals, electronic transport in a magnetic field is characterized by the motion of electrons along orbits on the Fermi surface, which usually causes an increase in the resistivity through averaging over velocities. Here, we show that large deviations from this behavior can arise in density-wave systems close to their ordering temperature. Specifically, enhanced scattering off collective fluctuations can lead to a change of direction of the orbital motion on reconstructed pockets. In weak magnetic fields, this leads to linear magnetoconductivity, the sign of which depends on the electric-field direction. At a critical magnetic field, the conductivity crosses zero for certain directions, signifying a thermodynamic instability of the density-wave state.