My Dogs is showing aggression to submissive dogs only

Hi
I wonder if anyone can offer some advice.
My SBT Roxy has always been well socialised and always v. good with other dogs and has always enjoyed the company of other dogs. However just lately she is showing signs of aggression but only to submissive dogs, like a kind of bullying. She does not show this behaviour to more dominant dogs.
A bit of history - Roxy will be 5 this August. She has had two lots of tplo surgery, so over the last 2.5 years has had her walks and socialising very limited. She recovered from her last TPLO early this year and is now on full walks. She never used to show this 'aggression' before her operations and I am wondering if she has lost her ability to socialise? If so why is she only like this with submissive dogs.
She shows this sign of 'aggression' on and off the lead. She has not actually bit another dog but lunges and barks which has always resulted in the other dogs running off. If she has been off the lead she starts to runs after them but always comes back as I call her back.
On a walk earlier in the week (we have kept her on the lead because of this behaviour) a playful, yet submissive dog came up to her and she showed the agressive behaviour immediately. The dog then ran off. Literally two minute later a much larger confident dog came bounding over to her and she submitted immediately by rolling over (which she always used to do with most dogs) and then she had a play with him, NO agressive behaviour shown at all. Two totally different behaviours.
Has anybody come across this behaviour with their dog/s? Obviously we are concerned if this gets worse and she actually bites another dog. We are also obvisouly concerned because of her breed.
She is absolutely fine with people.
Any information/advice would be a great help.
Thanks

I'm sure someone will be along soon with some advise hang in there

I have a Springer that would bully submissive dogs (and people) given the chance. He interacts just fine with confident dogs but the slightest hint of fear or submission and he turns into an an aggressive bully by chasing, biting and trying to hump them.

It is fairly easy to manage as he has a VERY strong "Leave", and if told will do a big loop around the other dog. If I don't trust the other dog to stay away from him I will recall him and make him sit with me until the other dog has passed by.

I`m not quite sure what you mean by submissive dogs. Do you mean fearful dogs? If so, that could be making your dog uncomfortable so she is trying to make the other dog go away.
In any case, it`s behaviour you don`t want - so a few things you could do is...
train her round other dogs. This is good for getting her to respond to you better.
Play with her more when other dogs are around. I don`t really want my dogs pestering others so I prefer they get their fun with me.
Work on your recall.
Introduce avoidance: next time you spot a possible target, call your dog, reward, detour round the `threat`, reward. This is the behaviour you want tbh.
I teach a `watch me` command which makes my dogs look to me. It`s very useful for distracting them.

hello there ,im pitbull guy and i see this as nothing more normal behaviour. sounds like all my dogs,its about how you curb the dog to fit your needs and to set boundries.sounds like you dog is exerting her confidece,when she can,submits when she needs to.the trouble comes when 2 dogs are confident and neither will submite.and some dogs dont care and go with the flow,no two are the same and theres no real way to get a dog to like an other dog,its how you manage that, my wife left 2 male pits for 30 seconds alone,it was not good,both are alive and seperated when im not there.when im with the both of them i have to curb there whant to callange.its taxing and for real.will they learn to like each other? no,but they tollorate each other ..when im there with them.iv got one dogo who will exert her confidence,and if i give it a min she will whant to play,never will she strike out and fight she will just show the other dog how confident she is and only once have i seen her back away,it was her brother, it happend in a flash ,she walked away and it now the way it is.the play and work together and there is an order i can see in my dogs when we speek, confidence ,not domanance.and personality plays as well...all of this post is of my opion and experience..respectfully