David Looser wrote:

Indeed, excessive bandwidth of the incoming signal.

Excessive slew rate. There really is no other adequate way
of saying it. Not excessive bandwidth, or amplitude, or
combination of the two, but slew rate, pure and simple. A
signal of a particular frequency has a max slew rate which
depends on its amplitude, and vice versa, so neither
frequency nor amplitude alone define a slew rate. Although a
particular combination of frequency and amplitude defines a
maximum slew rate, it isn't a unique definition because it
could be the same for any number of other combinations and
waveforms.

But as Jim showed a simple passive filter on the input to
the amplifier solves it.

Not if it's bad enough so that it can occur at audio
frequencies, or frequencies otherwise necessary for the
satisfactory operation of the amp. Then the cause of the
limiting must be dealt with.

Ian