I had that happen to someone on the team this week. If someone is using a template you were once assigned to or copying a previous project, it's possible that you're being added to the new one automatically.
No problem. And I should've added this: if you look at the "People" tab on the left side navigation of the project, you can see everyone who has been added to the project. You can also remove people from there if they are no longer working on it but still receiving notifications.