maybe its the other way around then, maybe the non dominant dog is testing the dominant one, but subtley so you havent noticed, but its making tilly cross. maybe tilly is under the weather and the other dog senses it???
the only time i have similar problems in my house is when one of them gets poorly - in fact, i got very upset when my westie became ill and my normally very docile jrt started bullying him
i know its nature, but its horrid
is tilly due at the vets for anything - maybe jabs? you could get your vet to check her over just incase.......