  Production of pions in proton-nucleus (p+A) reactions outside of a
kinematical boundary of proton-nucleon collisions, the so-called cumulative
effect, is studied. The kinematical restrictions on pions emitted in backward
direction in the target rest frame are analyzed. It is shown that cumulative
pion production requires a presence of massive baryonic resonances that are
produced during successive collisions of projectile with nuclear nucleons.
After each successive collision the mass of created resonance may increase and,
simultaneously, its longitudinal velocity decreases. Simulations within Ultra
relativistic Quantum Molecular Dynamics model reveals that successive
collisions of baryonic resonances with nuclear nucleons plays the dominant role
in cumulative pion production in p+A reactions.
