ARGH. My normally calm puppy was being a nightmare like that Rottie on a walk last night. The possible difference was that her posturing and vocalizations were all playful, not hard barking.
She saw this random other dog playing with a lab on dropped leashes as we walked past and wanted so badly to join in, but she wasn't invited. So for the rest of the freaking walk, she had it into her head that this other dog should be approached to play. Had it been a normal sidewalk walk, I'd have just aborted mission and changed routes, but it was a lake trail so we were all going the same direction in a circle. And I was pretty close to the end of my circle, so turning around wasn't a great option either.
This other owner was...less than pleased despite the fact that I was clearly trying to use each episode as a teaching moment. Each time she pulled toward the other dog and puppy-yipped and play bowed, I corrected for pulling, distracted and created a visual block with my own body, and praised for an obeyed command (thankfully no matter how excited she is, "sit" is very reliable) before resuming a "let's walk" or "heel." That other dog's owner probably thought my GSD was a horrible uncontrolled dog. She gave me a lot of dirty looks - and I don't really blame her, how could she know my dog truly just wanted to play?
Good on Bella for responding so well! Hopefully the Rottie was also just having an off night!