Advice needed: Aggressive behavior from Pyrrha on walks

So, since we’ve been fostering, Pyrrha has displayed a totally new behavior on walks with our fosters. She has never done this with us before, not even a shade of it.

Pen Park with Laszlo
Pyr walking trails with my sister and brother-in-law.

Here’s the scenario:

We are walking with Pyrrha and the foster (whether it was Brando or Laszlo) in the neighborhood. Another leashed dog and its human start approaching us. When the dog gets close enough to pass by us, Pyrrha FREAKS out. She lunges at the dog, barking ferociously, hackles up. I pull her back with all my might, utterly stunned and shaken. (And embarrassed!)

I am pretty sure that this new behavior is not a fear display. In the early days, her fear exhibited in her hackles up, tail curled under, ears back, lips curled up, slinking away, quiet growling; THIS is lunging forward, vicious-sounding barking, full body thrown at the other dog. Although it may still have its roots in fear, it does not look like a fearful display; it just looks outright aggressive.

I don’t know what this means. Someone suggested that she’s protective of the foster. I guess this could be, but I’m unsure. I need to walk her on her own, without another dog, and see how she does. Again, I have never, never seen this before and I don’t know how to handle it. She has now reacted this way to passing dogs on walks with both Brando and Laszlo. (As a side note, she hasn’t flipped out with every single dog we see; it’s only certain dogs. Last night, she freaked only after Laszlo had barked at the other dogs.) It’s only with dogs, too. We passed some unusual-looking people, children, kids on scooters — nothing.

I started to question my posture and energy, but I don’t feel like I was tensing up, because normally, when other dogs would pass us, she was SO happy! I wasn’t nervous when other dogs passed us. She’d pull me to them and start play-bowing. I just had no idea this behavior even existed inside her.

Any advice?

What do you think could be causing this behavior? Ever seen this in your own dog (a totally surprising reaction in a familiar environment with familiar stimuli)?