Sadie has her moments. She can be so sweet and loving and then out of nowhere she gets aggressive with Max or Lita, most the time for no reason, just out of the blue. I can tell when it's going to happen, she gets this look on her face and then she starts with a low growl and then just goes after Max or Lita. If I can catch her when she gets that look and starts the low growl I can sometimes redirect her and avoid the aggression but once she starts her attack on Max or Lita I have to put her the kennel to calm her down. I don't know what's causing it, she can play so nice with Max most the times but at least once a day she goes into this aggressive mode.
We have a dog that was showing random agression to our new basset. Bo, a pointer mix, will usually play very nicely with Maggie. Every once and a while though he would snarl at her relentlessly and we couldn't figure out why since there was no toy or food to protect. As it turns out, he was protecting me. He only acted like that if I was sitting with them when she wanted to play. This may not be your problem but it did teach us that dogs see things very differently so maybe your pup is reacting to something you haven't picked up on.