I frequently work with fearful or aggressive dogs. They are grouped into the same category because aggression always has a fear component unless an animal is hunting for food. Fear aggression manifests itself in warnings to tell the other dog or person to stay away. A bite is a more intense warning if the other warnings, such as growling, go unheeded.
Dog Training Blog
One of the more frequent issues with shy dogs is to be afraid of getting petted by strangers. When dogs are getting petted the person is close to them, they are looking at them, they are looming over them and then they touch them. These are all potential triggers for anxiety or aggression. If your dog is shy, you should help her get comfortable with people to avoid escalation of anxiety, which could potentially lead to aggression.
It is so important to gently handle your dog throughout his or her life. I received a typical call a while ago from a client that is afraid because her dog growls when she puts on his harness.