guarding the door

[kimberley1453]

Hi, My 8 year old german shepherd has been with me 5 years. I have had piano students come and go through the front door all of this time. Heidi always greets and is sweet. Suddenly at the end of February she started barking in a very aggressive manner at my students. Not all of them and not the same ones each week. She also is friendly and quiet to some people and suddenly after 5 minutes of petting by the student she starts with the aggressive barking/. She takes an aggressive stance and once last week circled my student. I immediately step in and stop the situation. What is going on and how can I stop this behavior.
Nothing in her life has changed. She leaves the house, walks and eats all on command. I try to maintain the alpha position. What should I do?
Kim

[KOPsarah]

note: to see the reply to this post please view sudden aggression post