  Observations of the SNR Cassiopeia A (Cas A) show asymmetries in the reverse
shock that cannot be explained by models describing a remnant expanding through
a spherically symmetric wind of the progenitor star. We investigate whether a
past interaction of Cas A with an asymmetric circumstellar shell can account
for the observed asymmetries. We performed 3D MHD simulations that describe the
remnant evolution from the SN to its interaction with a circumstellar shell.
The initial conditions are provided by a 3D neutrino-driven SN model whose
morphology resembles Cas A. We explored the parameter space of the shell,
searching for a set of parameters able to produce reverse shock asymmetries at
the age of 350 years analogous to those observed in Cas A. The interaction of
the remnant with the shell can produce asymmetries resembling those observed in
the reverse shock if the shell was asymmetric with the densest portion in the
nearside to the northwest (NW). The reverse shock shows the following
asymmetries at the age of Cas A: i) it moves inward in the observer frame in
the NW region, while it moves outward in other regions; ii) the geometric
center of the reverse shock is offset to the NW from the geometric center of
the forward shock; iii) the reverse shock in the NW region has enhanced
nonthermal emission because, there, the ejecta enter the reverse shock with a
higher velocity (between 4000 and 7000 km/s) than in other regions (below 2000
km/s). The asymmetries observed in the reverse shock of Cas A can be
interpreted as signatures of the interaction of the remnant with an asymmetric
circumstellar shell that occurred between 180 and 240 years after the SN event.
We suggest that the shell was, most likely, the result of a massive eruption
from the progenitor star that occurred between $10^4$ and $10^5$ years prior to
core-collapse. We estimate a total mass of the shell of the order 2 Msun.
