My parrots ruffle their feathers as emotional expressions (when they're pissed/ puzzled, etc.)...not unlike when the hair stands up on a dog or cat's back. So it might be that the plates were a communication device?
I have parrots, too, and I agree with your line of thinking. Since birds and dinosaurs have a close evolutionary relationship, I would not be surprised if some bird behaviour would be found in dinosaurs as well.