Why would my 125lb. bullmastiff RUN from a stranger?

My husband was clearing some bushes behind our privacy fence and our 2 bullmastiffs were going crazy barking at him (they couldn’t see him, just hear something back there). He peeped his head over the fence just enough to see them (but they couldn’t tell who he was probably, just could see his forehead). Our female went wild barking and growling. Our male barked a few seconds while backing up, then turned around and ran to the back door and scratched to come inside! Why would a 125lb. protective breed dog do this? His behavior surprised us both.