  It is commonly believed that azimuthal anisotropies in relativistic heavy ion
collisions are generated by hydrodynamic evolution of the strongly interacting
quark-gluon plasma. Here we use transport models to study how azimuthal
anisotropies depend on the number of collisions that each parton suffers. We
find that the majority of $v_2$ comes from the anisotropic escape of partons,
not from the parton collective flow, for semi-central Au+Au collisions at 200A
GeV. As expected, the fraction of $v_2$ from the anisotropic particle escape is
even higher for smaller systems such as d+Au. Our transport model results also
confirm that azimuthal anisotropies would be dominated by hydrodynamic flow at
unrealistically-high parton cross sections. Our finding thus naturally explains
the similarity of azimuthal anisotropies in small and large systems; however,
it presents a challenge to the paradigm of anisotropic flow.
