We have the same issue, and I think it's both prey drive and annoyance. Topaz is English Mastiff x Weimaraner, so the prey drive is very strong in her. However, I have walked by (on leash) small dogs with her and as long as they are quiet and don't act like they are the boss (which they usually do), she is fine. She will still raise her hackles, because she raises them about 45 times a day, but she won't "React" otherwise. But if a small dog barks at her, growls, or stiffens, she will be totally silent and just lunge or zone in on them. She has to be boss. She hates bossy dogs.
Topaz is also very possessive. If another dog has something, Topaz will do whatever it takes to take it away from that dog. She is a bully for sure. Most big dogs will give it up and there isn't a problem, but if the other dog wants it too, then we have a fight. And if that dog is hyper or very active, then Topaz doesn't like it. So she feeds off energy given back for sure. But I also think there is prey drive in there as well