Why do I have an orange line in my Github 'Open pull request' page?

The orange line means that it was highlighted. Check the URL of the Pull Request link you are clicking to see if there is something like #L5 at the end. It is nothing to worry about.

It's just line highlighting, click on the line number 5 and it will disappear.

Your url should also be ending with #L5. This is mainly for url sharing purposes.

In this case you probably misclicked it yourself.