Murphy becoming anxious/aggressive around dogs.

[Sue, Murphy and Bella ()*o*()]

Murphy is a rather anxious dog, but otherwise friendly and well socialized to humans and dogs. He had started about 2 years ago to growl and bark at dogs that come near me or Bella and this has slowly escalated to him chasing dogs away from her and then humping her, I guess to show his dominance and ownership of her???
It has just gotten to the point of needing to separate him from other dogs that visit, and to stay hyper aware at the dog park so that I can step in between him and other dogs to walk him back away from the other dogs and to redirect him. I give a verbal correction and he does back away easily, however it is taking longer to get him to give up the barking and aggressive growling and it tends to excite the other dogs until I'm in a mix of barking lunging dogs. And other owners are appearing anxious around him and their dogs as well, from all the noise.
Im not sure what it is that is making him so uncomfortable and guarded now around other dogs and more unsure as to what to do about it. Is he getting more anxious as he gets older? Is he becoming more protective? Possessive? I feel bad for him because he used to play with other dogs and its now more waiting for whatever is going to set him off than it is watching him play.
Any ideas or suggestions would be appreciated.

[Deborah Brown]

How old is Bella?  Is she still a puppy?  You describe her as submissive but maybe she is a bit fearful too?  Does she get overwhelmed by other dog's attention?   I do think there is something that Murphy is seeing or at least interpreting as a situation where he needs to protect Bella.

[Sue, Murphy and Bella ()*o*()]

Thanks for asking, she will be 4 in July. Yes, I think one could say she maybe a little fearful. She only gets overwhelmed when there are more than one dog trying to smell or chase her. she is ok with one. In that sense, I agree he might be sensing something. He does bark when dogs come near me too, but Bella usually hangs pretty close to me.