It happens cause the slope of the modulator is bending the phase of th main oscillator back towards zero(because PM is dc coupled so it will always adhere to zero unless you mess with even ratios)what happens if you was to force the middle section of a sine to zero?you get a saw wave or a bandlimited one in the case of FM8.why doesn't that happen when you modulate a carrier with a modulator?because you are using the slope(not the whole waveform)of the sine to phase modulate another sine.when it reaches zero the phase nowhere to go but back up and down again and it keeps squeezing in harmonics between the oscillation rate of your fundamental.this is known as phase "wrapping'it all works a bit similar to oscillator sync hence why when you modulate a saw wave by a saw wave or a sine by a saw wave you get a shift of its formants..if you window that with a triangle,you basically have what a "formant"oscillator effect does on Serum,MASSIVE etc