<p>The ejection of relativistic outflows is the most spectacular consequence of accretion onto compact objects. It is powered by the interplay of gravity, particles and magnetic fields. The microquasar SS 433, one of the most exotic binary systems in the Galaxy, has powerful precessing jets. In these outflows, radio polarization measurements unveil a complex magnetic field topology that becomes parallel to the bulk velocity direction. Although the physical origin of this field topology remains unclear, it has been suggested that it may be linked to the underlying jet morphology. Here we investigate this intriguing connection. Using state-of-the-art numerical simulations that model the evolution of the jet magnetic fields, we show that the observed field orientation could naturally arise from collisions between discrete ejecta propagating with slightly different velocities on subparsec scales. These prompt interactions also lead to the formation of elongated plasma bullets that are dynamically more stable and, therefore, will more probably propagate larger distances without disruption. Our results indicate that discrete, interacting ejecta provide a plausible and self-consistent explanation for the observed magnetic-field alignment in SS 433 and offer new insights into the jet-magnetic-field coupling on subparsec scales.</p>