Step 1: Recall the meaning of compressor surge.
Compressor surge is an unstable operating condition of a compressor.
It occurs when the compressor cannot overcome the back pressure imposed on it.
Step 2: Identify the effect of excessive back pressure.
When the back pressure becomes excessively high,
\[
\boxed{
\text{the flow reverses through the compressor, causing surge.}
}
\]
Therefore,
\[
\boxed{\text{Flow reverses due to excessive back pressure}}
\]
is the correct answer.
Thus,
\[
\boxed{(C)}
\]
is the correct answer.