It may be something that your neighbours or neighbours dogs are doing that makes Banjo bark, or maybe kids on bikes riding past, a cat climbing on the fence, people taking little kids into their backyard to play?
We used to have a rottweiler next door that growled every time we were in our back yard, and whenever it heard my dog walking or moving out there. This of course made her bark at the rottweiler - those neighbours then complained to the council about our dog, the council came out and told use we had to use one of those electric zappy collars on her and if she didn't stop barking we would have to have her put down. We did what they told us obviously but then our neighbours moved anyway - after that no more listening to the neighbours dog growl at us and obviously no more barking from our dog!
Btw Banjo looks so cute in your signature, like a little fox