I agree that you should have someone come in to observe your dogs. The problem you are describing is called Resource Guarding. it is sometimes hard to see who is the instigator. This is when someone who knows how to look for the triggers can help.
If you google resource guarding, there is lots of help for you. Hope this will help. This is a problem that needs to be addressed, ,if not it will only get worse.
Bree 12 year old GSD
Keesha 8 year old GSD
Cyrus 7 year old GSD
RIP: All my angels.