Of course there is a reason. But that reason isn’t always “abuse”. That has become the mantra these days, to always blame the human, but it’s wrong.
Many dogs turn. Some possibly because of tumors (don’t know if they checked for that), but either way, they can become dangers all on their own. Some can be genetically inclined for this. Not shocking in a (working) Mal.
I have been around and raised dogs, specifically big dogs for about 30 years. Every time I’ve seen a dog turn, that’s every time with one exception, it was because of the handler/owner. The only exception to that was a dog that had a brain tumor and became very violent.
I’ve brought more than one dog back from abusive situations and it’s not difficult to do if you just have the patience and LISTEN to what the dog is trying to tell you.
I’m sorry but anyone that puts down a healthy dog needs an @ss whipping bad.