Hi there.
I also have a two year old working cocker bitch. She too can display aggressive behaviour to some other dogs.
Like yours, ours was heavily socialised from day one and didn't have any problems for the first year or so, but now she will have a right go at some, not all dogs. I have concluded:
-She is from a long line of ft champions who were just workers-not pets and so is very highly strung (she's been hard work).
-She has the cocker guarding tendency.
-She had quite a few bad experiences in early life (got attacked quite badly by a cockerpoo when she was tiny and has had all manner of rumbles and nips since)
-She is now a pretty confident 'adult' who knows her own mind.
I think all of these things come into play. I find that she will now decide who she thinks is appropriate company, and if you are rude, jump all over her (or me) or come near her ball without invitation, she will let you know. I also think that these types of cockers are not stupid-they know they are small, so they feel the need to make a bigger, scarier noise to get their point across.
Don't get me wrong, my dog actually has lots of friends, some of which she is even happy to share her balls with, but they are dogs that she knows and trusts and they always greet each other politely every time they meet. I also let other people's dogs into our house. It is my house after all.
Watching my dogs 'aggression' is interesting. If you stay calm and watch yours next time it does something you might see the same thing.. lots of noise, lunging towards the other dog, maybe even teeth out and mouth open, but if I look at mine, it is all done kind of sideways. I don't think there is any real intent. I've seen my dog catch things. If she wants to catch something in her mouth, she will not miss!!!
To me this is just a dog saying **** off out of my space ok? Our dog has never bitten another dog and I don't think she would.
When I saw the first instance of this behaviour I must admit I was horrified but if you really look at it it does look much worse than it is. I am lucky, as where I walk my dog is full of nice people who understand dogs, so I have had the 'opportunity' to bump into these dogs that she's not so keen on, repeatedly which has meant I've been able to really look at the behaviour.
I have found that you can easily override it.
One dog she kept having a pop at was( jealousy in that case I think as Charlie the bichon frise/poodle first got it after I picked him up!). Next time they approached she started to have a go at him and I just interrupted her- just said 'hey' and called them both over, made them sit, and gave them both a biscuit. They then proceeded to spend ten minutes doing tricks together! Like a performing dog team.
There are others that she can be funny with but I now just interrupt it by asking for something different. In this way, and with the help of the other dog owners keeping their cool too, we have established who she's funny with and who she's not. Sometimes excitement can spill over into a form of aggression as well but with an interruption and on the odd occasion putting a lead on to show shes gone too far and is not in charge, it's all perfectly fine. There were 4or 5 Dogs that mine was funny with but it's all fine now-I just watch and if I see any signs I just just warn her to behave herself.
Sorry for the essay but it's quite a complex issue and in summary I think the key is to keep calm and stay in charge.
I would jump on my dog if I thought it was hurting someone else's pet but equally I think if you can manage the dogs off lead it is preferable to just immediately curtailing them both as this could make them act differently in itself. In our case, I think our dog is learning that even if she doesn't like a dog, there are other ways to behave around them and its not her job to go telling people off. We ate all still off lead and all the dogs are fine with each other now, with a little reminding here and there from us hooomans!
Best of luck - I'm sure it'll all be good.