A stray cat comes to my screen door and my cats start fighting each other1?

shirae780 asked:

My 2 cats are about 8 years old, neutered, and are brothers. I don’t know if the stray is a boy or girl. The first time it came around the 3 of them fought thru the door. Now, even it it comes near the steps my cats will start fighting each other! Its really bad fighting. They rip hair out! They start hissing and staring each other down. It lasts for about 10 minutes. Why are they turning on each other?

Tom and Jerry