  We investigate the observational signatures and physical origin of
ram-pressure stripping (RPS) in 63 massive galaxy clusters at $z=0.3-0.7$,
based on images obtained with the Hubble Space Telescope. Using a training set
of a dozen "jellyfish" galaxies identified earlier in the same imaging data, we
define morphological criteria to select 211 additional, less obvious cases of
RPS. Spectroscopic follow-up observations of 124 candidates so far confirmed 53
as cluster members. For the brightest and most favourably aligned systems we
visually derive estimates of the projected direction of motion based on the
orientation of apparent compression shocks and debris trails.
  Our findings suggest that the onset of these events occurs primarily at large
distances from the cluster core ($>400$ kpc), and that the trajectories of the
affected galaxies feature high impact parameters. Simple models show that such
trajectories are highly improbable for galaxy infall along filaments but common
for infall at high velocities, even after observational biases are accounted
for, provided the duration of the resulting RPS events is $\lesssim$500 Myr. We
thus tentatively conclude that extreme RPS events are preferentially triggered
by cluster mergers, an interpretation that is supported by the disturbed
dynamical state of many of the host clusters. This hypothesis implies that
extreme RPS might occur also near the cores of merging poor clusters or even
merging groups of galaxies.
  Finally, we present nine additional "jellyfish" galaxies at z$>$0.3
discovered by us, thereby doubling the number of such systems known at
intermediate redshift.
