Why files I haven't touched become merge conflicts after pull

I was working on a branch. I made some changes to the file person.cs

that I wanted to apply to the employee's branch. I worked on his branch last week, but recently I have been working on my branch.

I switched to his branch and reached over to get his latest changes. After pulling, I got merge conflicts in a people.css

file that I didn't even open.

Why should I have merge conflicts?