  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 30m telescope, and a
spectrally-resolved [CII] 158 $\mu$m map observed with the SOFIA telescope
towards the massive DR21 cloud. This traces the kinematics from low- to
high-density gas in the cloud which allows to constrain the formation scenario
of the high-mass star forming DR21 ridge. The molecular line data reveals that
the sub-filaments are systematically redshifted relative to the dense ridge. We
demonstrate that [CII] 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 sub-filaments 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 pc 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.
