It's a very long time since I had to think about basic propellor theory but I'm surprised you prop drivers don't know why this happens.
I think you'll find it has to do with "TAS/IAS relationship". You'll need to go and look this up as i've forgotten the detailed explanation.
In practical terms, at high altitude for a given IAS an uncontrolled (ungoverned) prop will over speed and the result can be the separation of prop and engine with disastrous results for the airframe.
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