Both dogs are female? Female on female aggression can be really ugly and they may or may not get over it. The sniffing around the door and attacking again is not good.....I've seen that exact same behavior with two of our females. It was one sided aggression. Those two dogs remained separated forever. Not even in the same room together. Yeah there's ways you can try to reintroduce them, but with a big size difference its an accident waiting to happen. Sometimes all it takes is for one female to just give a little growl at the other female, or playfully nip, or some other perceived slight, and its on.
The fixating on a spot on the wall, reflections, shadows etc tends to be neurotic behavior, at least with german shepherds. Could be genetic, or the dog isnt getting enough mental/physical stimulation. Sort of like obsessive tail chasing.
Like the above poster said, theres really no way to tell what set the dog off on the smaller dog without seeing it.
That being said, my wifes black lab will tear into a small breed dog if it runs around her. She's fine with bigger dogs, just something about the small breeds sets her off when they run around her.
She's a good dog in just about every other way though, so we just keep her away from small dogs. For our living situation its just not worth the effort to try to "fix" it.
Dogs get in fights.....some females can remain enemies for life, even if one submits and tries to appease the other, the other may not accept that.
Shepherds like to chase things.....some of them can go into a zone while doing it..... There are technical terms for these things you often hear associated with german shepherds. Clear headedness is one.....like a dog that starts chasing another smaller dog, it goes into that zone, if its not a dog with a clear head, which is genetic, and of varying degrees, it most certainly can progress from just the chase, to the catch, and so forth. Its almost like the dog is so focused on what its doing at that moment it cant control its own impulses. Its impulses overpower its "logical" brain for that time. Of coarse none of this may be the case here.....it really could be as simple as the smaller dog did something to [email protected]
@ off the shep mix, and she just wasnt having any of it, and is one of those dogs that just wont forgive the little dog.
Keep in mind also, what does sometimes happen, is the dog everyone thinks is the victim, actually is the one that starts the fights......but the stronger dog finishes it and gets all the blame.
None of this means the dog is bad. It just means more work for the owner, extra management, and limits where and what you can do with the dog.