WSJT-X overdriven signals

Recently I was once again qrv in FT8 on 6m. It was nice conditions with some DX.
Suddenly the band was completely cluttered with a signal.

Not enough that this station was cluttering the band, additionally a harmonic on 70MHz could be detected.

How can something like this happen?
You are using a very inferior transceiver e.g. IC-7100
The WSJT-X sound setting LF is turned up too high (should not be more than 50%, pay attention to ALC)
The power amplifier used has no or a defective bandpass filter
The “fake it” setting is not activated in WSJT-X.
The DXpedition mode “stream” is used with MSHV.