My roommate's dog does this to my GSD when someone walks by the fence.
IMO I think she is redirecting her aggression. She get's frustrated and lashes out at the thing closest to her which is usually my GSD or my roommate's other dog.
Sinister "The Prince of Darkness" 4 yr old black male GSD 3.11.09
Draco "Dakota's Unbreakable Vow" 5.5 mth old male Dalmatian 6.30.13
Cats: Chaos, Mayhem, Monster, Wicked