What is the probable cause of "flat topping" (non-linear operation) of an amplifier in an SSB transmitter?

"Flat topping" occurs when an amplifier is driven so hard that its output voltage hits its maximum physical limit, clipping the peaks off the audio waveform (making them look flat on an oscilloscope). In a Single Sideband (SSB) transmitter, this is most commonly caused by turning the microphone gain up too high. This severe distortion creates unwanted, wide sidebands known as splatter. Defective low-pass or sideband filters affect frequency response and sideband suppression, not amplitude clipping, and while an insufficient power supply is bad, excessive audio drive (mic gain) is the direct and standard cause of flat topping.