CMV: defensiveness is indicative of guilt

This concept seriously makes no sense to me, it’s a strange double standard in my opinion.

For example, think about being physically attacked/assaulted. General consensus would be to fight back. To defend yourself. You probably won’t just idly accept blows without doing something.

So how is defending yourself against untruths in an emotional/personal type of way somehow a red flag of guilt?

You can bet I’m going to retort the reason why the accusation(s) being hurled at me have no validity.

Make it make sense.