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.