By design, Drafts tries to always be ready with a new draft when launched to make it easy to capture thoughts quickly.
There are times, however, when you jump in and out of Drafts to copy some text from an email, or a URL from Safari, etc., and it would be annoying to have a new draft pop up when you return immediately to Drafts.
To alleviate that problem, Drafts has a timer and will only create a new draft automatically when you have not returned to the app for a certain period of time. The default time for this timer is 60 seconds, but it can be adjusted to 30 seconds, 5 minutes or Never via the "Create new draft when returning to app after..." setting.
If you set it to "Never", Drafts will always return to the last draft you were editing, and you will have to create a new draft yourself.