A bully in the Back yard!?

I was wondering what I can do to stop my dog's bullying behavior! I have a 1 and 1/2 yr. old Rottweiler and a 4 yr. old Collie. The collie was a rescue, he came from a really bad situation and it took us two years to get him to be somewhat himself. When we got our rottie puppy, the collie was protective and took good care of him. They loved playing together in the back yard. once he got to be a certain height I noticed that my rottie grabs the collie's tail and would pull hard! Then he would bite the hair on his and around the collar. Now a days he still does these things, but he also grabs the collie's mouth, Once the collie gets mad and bites him, the rottie comes back and keeps pestering him!! But only outside and when they play inside the house. there is never any aggression with food or water, nothing like that so I don't know what to do?? When the collie eats, the rottie leaves him alone never even bothers him! And why does my rottie keep his nose in the collie's especially when he is going potty. It's like he has to sniff everything! Now my mother-in-law's dog is picking on the collie!! He barks in his face twenty-four seven when they are outside!! I don't get it!