How can I turn off music automatically starting after phone call in iPhone
My guess is that it is a (typically) poor implementation in the bluetooth headset.

I have the same issue with two different cars that are connected to the iPhone via bluetooth. I guess the developer writing the code for the device couldn't imagine that anyone wasn't listening to music, so he hardcoded to start the music after the call ends.

I could not find a solution in many years, and have now disconnected my iPhone from the car for that reason. Obviously not useful for you, as a bluetooth headset that is not connected is useless (whereas the car still drives).