Direct answer
You over-explain when the explanation has stopped informing the other person and started working to keep them from thinking badly of you. At that point you're not communicating a decision: you're asking permission to have made it.
The difference between informing and justifying
Explaining can be care and consideration. Sharing the reason for a no can be generosity toward whoever asked. The problem starts when the explanation changes function. It stops informing and starts serving one purpose only: keeping the other person from forming any negative opinion of you.
It's a subtle shift, but a decisive one. In the first case, you communicate. In the second, you negotiate. And what's up for negotiation isn't the person's request, it's your right to say no without losing their good opinion.
Where power is handed over
When a decision only holds if the other approves, you've given them something that was yours. This is the mechanism of handing over power: an external gesture, someone else's approval, comes to decide whether your choice counts. Holding a decision without turning it into a defense is where the choice becomes yours again.