Socialization issues like this it is good to watch the three lecture videos.
You are going to want to resist the urge to shelter the dog for the safety of others. This is generally a sign the dog needs more socialization and that it should be made a training priority over everything else. At her age it is easier to socialize than it will be later.
When a dog reacts like this assuming it isn't in a playful way (we'd have to see the dog to know for sure) it is fear based. There might have been signs of subtle avoidance behaviors before but now that the pup is a little bit older and the hormones are starting to rage he gets a little more confidence and decides to pick fight over flight or avoidance. You really want to start socializing a ton now especially because you have one that is leaning towards the fight side of things. There is strong potential for her to become a fear biter.
The dog is hit or miss because there is some socialization to certain types of people, but people that fall out of the age group she is socialized to or who are wearing unusual things shes not used to like big hats or ski masks or carrying an umbrella still cause a fear response.