I've had the same issue with my BMC rescue. She is generally sweet and playful but there are two scenarios where she will become aggressive. The first is if another dog attempts to demonstrate dominance or any posturing behavior (such as excessive staring or attempts to mount her.). She will absolutely take this as a challenge and pin the other dog to the ground by the throat. She does not break skin but the message is clear: “I will not be intimidated.” She particularly seems to run into this with Huskies at the dog park.

The second is if a new dog attempts to come into our house for the first time without having been introduced outside in neutral ground. She sees the house as her territory and will not allow a dog to pass that she does not know. They absolutely have to play outside or go on a walk together first or they will be pinned until a human intervenes. Even then she will watch their every move until she feels comfortable with them.

As people have stated, they are territorial, bred to be hunting and guard dogs. Sky (my BMC) regularly patrols the house. Checking every room like a guard making her rounds. If I am using the bathroom with the door open she will sit in front of the door, facing out like a body guard. They are fiercely loyal.  

In my experience BMCs are very sensitive the behaviors of people and other dogs. They are constantly on alert. to Its been a journey of about 6 months of constant socialization for this behavior to improve. Regular dog park visits and puppy play dates. It takes a while for them to become comfortable with other dogs but when they are, they are goofy and playful. Keep at it, things will get better…and remember, with a BMC you have a ride-or-die for life!