What is a common cause of overmodulation of AFSK signals?

AFSK (Audio Frequency Shift Keying) works by generating audio tones in a computer soundcard or TNC and feeding them into the audio input of a transmitter (typically operating in SSB mode). If the volume of this audio signal is set too high, it overdrives the transmitter's audio and RF amplification stages. This causes clipping and overmodulation, resulting in splatter, distorted signals, and interference to adjacent frequencies. While "excessive frequency deviation" might sound plausible, it is technically a symptom of direct FM overmodulation, not the root cause in an AFSK setup. Bit errors and retries are consequences of a poor data link, not the cause of the transmitter's overmodulation.