  The prototype dwarf nova SS Cyg unexpectedly exhibited an anomalous event in
its light curve in the early few months of 2021 in which regular dwarf
nova-type outbursts stopped, but small-amplitude fluctuations occurred only.
Inspired by this event, we have performed numerical simulations of light curves
of SS Cyg by varying mass transfer rates and varying viscosity parameters in
the cool disk. We have also studied the effect of gas-stream overflows beyond
the outer disk edge in the light curve simulations. We have confirmed that the
enhanced mass transfer is unlikely responsible for the 2021 anomalous event and
its forerunner. We have found that the enhancement of the viscosity in the disk
may reproduce the forerunner of that event but may not be enough to explain the
2021 anomalous event, although the latter result might be particular to our
thermal equilibrium curve used. Within our simulations, a model of the gas
stream overflow with a slightly higher mass transfer rate than that of our
standard model reproduces light curves similar to the 2021 anomalous event. We
suggest that the gas-stream overflow is necessary to reproduce that event. The
gas-stream overflow may also be responsible for the abnormally high X-ray flux
during the normal quiescent state in SS Cyg.
