My 8 year old Labrador ( Bitch )is the softest dog you could meet . She loves to be stroked & petted by everyone . But the bloke down the street walks past with his two Rottweilers , and my lab goes mental . If she's in the front garden , she will physically try to get at the Rotty's . All other dogs , she wants to play with .
I think dogs just get a look from other dogs, we know as humans that someone can just look scary without any justification. My dog hates some dogs so I just assume they are communicating in some way we don't understand. Try distracting her with play or food to take her mind off it and giving her a treat when she is calm.