Yes, I have seen that reaction to being protective, especially from Rotties.

My Ausie Cattle dog I had for a long time did that with a puppy. She was fixed and about 4 years old at the time. My dad (divorced from my mom) had to go out of town for a week, and asked my mom my dog from his house could stay with us. My mom said yes, and we brought the dog over. (less than a year old) Well, they got along great! When my dad came back after a week, my cattle dog, knew what was up. My dad went to go pick up the puppy, and my cattle dog started to growl and hover over the puppy. We obviously moved her out of the way and got everything straightened out.

It's amazing to see dogs take over that "motherly" instinct.