We had a chat with a behaviourist about our Welsh Springer recently. She said that dog hierarchies are very fluid and change on a minute by minute basis. Your springer sounds like she’s the boss and is reminding him of the fact. Ours does this to our much older cocker, with lots of noise and dominating.
The solution we were given is to reinforce your springer’s dominance by giving her everything good first e.g. walks, off the lead, food, petting, and think about other areas where she may perceive the Jack Russell as being favored e.g. being picked up, allowed on furniture. After a long period of calm we have it all happening again as the cocker has medicine and a special food as she’s been ill. It’s not much fun watching the old lady get flattened, but that’s just one of the joys of sharing your life with dogs…