For various reasons, a keyword in your campaign might be removed or declined.
How will you find out?
You will receive a notification email and/or you can have a look at your campaign alerts, visible on the "Dashboard" screen. You need to click "View all alerts" and then click on the one that informs you about the declined or removed keyword.
Where do you find the reason why it has been declined?
In the editorial status page you will only see the status, not the reason.
Just click on the keyword itself, you will then be directed to a very good explanation page with all details (which campaign and ad group, the reasons and suggested actions).