I think they are just being terriers! You have a combination of JR and Borders - both feisty breeds to say the least. This is just what terriers sometimes do - for example I look after two unrelated Border Terrier bitches - have lived together for eighteen months and play together like you've never seen. Quite out of the blue dog A launched a massive attack on dog B which resulted in a visit to the vet! Only thing that could be deduced by very experienced terrier owners was that dog B was digging bulbs out and dog A took exception to this. Really these people are proper country folk and have kept terriers for a thousand years. Any breed of terrier is feisty - they have been bred to display courage and fighting tenacity so your dogs are only displaying their natural traits.