Why is he doing that?
 

I've said before that Bob has come into the house and likes making himself at home. He has not clawed the furniture lately, and that's good. But what's not good is how he acts around Shadow. I try to keep him and the smaller kittens separated since Bob's a full grown cat and he's bigger and stronger than they are. But there have been some times when Bob's chased Shadow and I've been worried about her. And here's the thing: Shadow never did anything to him to make him go after her. He has also never gone after Sassy or Pretty Kitty.

Bob and Shadow rarely interact, but this is the second time that Bob's been kind of aggressive toward her. Bob made it into the basement and I closed the door so he wouldn't get too far. However, I didn't know that Shadow was with us. I called for Bob, but then I heard him growl, and Shadow came out with him behind her. I tried to catch Bob, but he found one of Ambre's old hiding spots. I left him, and Shadow was hiding somewhere else.

Bob finally got out of the basement and back in the breezeway, and Shadow came out afterwards, and she was all right, but how do I keep him from hurting her if they meet again? And why is he acting like that?