Abstract We present new 13 CO (1−0), C 18 O (1−0), HCO + (1−0), and H 13 CO + (1−0) maps from the IRAM 30 m telescope and a spectrally resolved [C ii ] 158 μ m map observed with the SOFIA telescope toward the massive DR21 cloud. This traces the kinematics from low- to high-density gas in the cloud, which allows us to constrain the formation scenario of the high-mass star-forming DR21 ridge. The molecular line data reveal that the subfilaments are systematically redshifted relative to the dense ridge. We demonstrate that [C ii ] unveils the surrounding CO-poor gas of the dense filaments in the DR21 cloud. We also show that this surrounding gas is organized in a flattened cloud with curved redshifted dynamics perpendicular to the ridge. The subfilaments thus form in this curved and flattened mass reservoir. A virial analysis of the different lines indicates that self-gravity should drive the evolution of the ridge and surrounding cloud. Combining all results, we propose that bending of the magnetic field, due to the interaction with a mostly atomic colliding cloud, explains the velocity field and resulting mass accretion on the ridge. This is remarkably similar to what was found for at least two nearby low-mass filaments. We tentatively propose that this scenario might be a widespread mechanism to initiate star formation in the Milky Way. However, in contrast to low-mass clouds, gravitational collapse plays a role on the parsec scale of the DR21 ridge because of the higher density. This allows more effective mass collection at the centers of collapse and should facilitate massive cluster formation.< Réduire