I think it is just down to defending something she sees as hers and doesn't want other dogs to help themselves to. My border collie bitch would be the same with any ball, not just hers, lol. She claims them as her own and god help any dogs that tries to take it.

I agree about maybe taking the ball away or making sure it isn't around when other dogs are.

Is she neutered? Maybe if she's done, it will help with the marking issue and the dominance with other dogs issue.
My BC bitch has always been a bit of a marker too but being as she is now 13, I don't think it is worth messing with now.