I am new on here and I just came across this thread. My 2 little dogs have had the exact same problem!! I have a Maltese (Daisy very docile and sweet) that is 2 and a Yorkie (Ginger - the aggressor) that is 5, and it is Ginger that had started bullying Daisy.
I don't know what in the world it is, but it is the exact same thing that has happened with me over the past year. It seems when the weather gets cold that is when Ginger starts acting differently toward Daisy and will attack her out of no where, for absolutely no reason.There's never been serious blood, thankfully it's never been too serious, just scratches, but it always scares me and I think it stresses Daisy out. Its so weird because one minute they are friends and playing and the next they could be fighting!
Reading what you wrote, sounded like my exact situation. I have yet to find the perfect remedy, it could still happen at times, but what I have found to work and what has helped them fight less was
1) giving Ginger her space during the day when I'm gone. I'll separate them and I think Ginger, with getting older and everything, would get either annoyed or frustrated with Daisy and just react in that way to get her to go away.
This has helped a lot, because by the time I get home Ginger is well rested, stress-free from being in her small space, and ready to play or walk with Daisy.
2) I'd keep a spray bottle around to spray them just as a shock to snap them out of it if Ginger ever tried to react to Daisy by going after her. I think this may have only worked because Ginger HATES the water bottle.. she will run from it, but she is also a little dog.
3)Anything strong smelling like tea tree oil gets them to snap out of a fight and relax. I think it just shocks them for a second and they calm down.
4)If I ever sense Ginger acting even the tiniest bit aggressive or she seems to be annoyed with Daisy I just separate her and put her in the room. She doesn't ever seem to mind, because I think the space is possibly what she wants.
I have no idea if any of these things could work for you. I've been to vets and trainers.. they are all healthy, and I can't get any answers for why this happens, but all I know is that those things have help me so it could possibly be worth a try? Hope this helps!! I know how frustrating something like this is to go through!!