Dog behavior question.

[TNDemo]

We have an approximately 9 yo beagle/basset and a 1-1/2 yo Boston Terrier. Both are rescues so I am not sure of their background. We have had the bagle for four years and the BT for 10 months. The BT seems to not be able to control himself with the bagle gets excited. It just freaks him out. If they sense we are going for a walk the bagle gets happy about it and the BT will nip at her heels and the back of her neck and just be a general nuisance. This happens whenever there is something that makes the bagle happy or excited. What is up with this? He can't seem to take his attention off of her. If there is a dog outside the fence and the bagle is barking and running at it the BT doesn't look at the outside dog, just focuses all his attention on chasing and nipping the bagle. Ideas? I imagine it is some kind of dominance behavior but it's annoying, especially to the bagle.