Why AFSK (audio frequency shift keying) works by feeding audio tones from a soundcard or modem into the microphone or line input of an SSB transmitter, so the audio level directly sets the drive level. Too much audio pushes the transmitter past its linear range, causing ALC action, flat-topping, splatter, and strong intermodulation products either side of the signal. The fix is to reduce the soundcard or sound device output until the ALC barely moves and the RF output is well below maximum.
Watch out Excessive frequency deviation describes overmodulating an FM transmitter, a different mechanism; with AFSK on SSB there is no deviation control, only audio gain. Retries and modem bit errors affect throughput and data integrity, not the transmitted waveform's linearity.
AFSK rides in on audio: too much audio in, splatter out. Turn the soundcard down until ALC barely twitches.