My cat is acting very funny?

So I live in an apartment and my boyfriend and I have an indoor/outdoor cat. He's been outside staring at a bush and growling for 20 minutes now. I was trying to call his name and he would just meow at me and stare back at the bushes. After a minute another cat came out of the bushes and walked away and my cat walked over to me, meowed, and walked back to the bush. Why in the hell is he acting so funny?
The reason I went outside is because he was growling. And when I called his name he was just growling into the bushes