when a bridge drives an inductive load the diode capacitance forms a resonant circuit during on to off conduction state. A small RF cap lowers this resonant f, which in turn reduces the E field EMI on the conducting wires and thus avoids emission issues in the AM band. As diode capacitance changes with reverse bias, the resulting noise without this cap might present an envelope burst of RF during turn off or just a pulse depending on wiring capacitance and stray leakage
