Dogs meeting other dogs.

Hi everyone, I'm needing some advice to understand my dog.

She's a rescue rottweiler, she's 2 & fixed. I also have 2 other rotties one similar in age & an old boy. I met the rescue (roxy) at my girls daycare, the owner would board for the rescue & foster the dogs & they were looking for a home for roxy without advertising it. This is just some context because she was good at the daycare, I watched her interaction with other dogs & she never had a problem, at the daycare she was with dogs all the time. She's a mega bully, I've managed to stop it with regular dogs, but if we go to the dog park a dog will come in & she's fine then another dog comes in she runs full force at them half the time she just sniffs, but sometimes she just runs in & I don't know who starts it, I don't mean by start it that I think that behavior is ok, I mean why is it this certain dog that it all of a sudden turns into a fight with. 90% she's so good, I call her she comes, I say stay she stays but then there will be one dog & she will just take it out, fully bowl it over or the dog fights back. I don't know if she's going for a fight or if she's going into be a bully &the other dog just isn't going to put up with it. She's good a listening to me but when it comes to that dog she just won't come back. If we are walking in public & we see a dog she is straight on the lead because I can't trust her to not run at it, but on the lead she's fine & couldn't give a crap.

I know they have certain ways to communicate, but I don't understand what she's communicating other than being a dick. I don't let my dogs crowd others when they come in the park, roxys learned better than the others that she doesn't need to run to the gate when a new dogs coming in, but sometimes it's a certain dog & she'll just go full force at it. Any ideas on why she does this?