I find that I keep doing this on my iPad. Maybe it is the five-finger pinch to minimize an app that is causing the problem. I think one finger partially swipes a message to the right which is one way to create a scheduled folder. The problem is: there is no undo for this action. Your only choices are to choose a scheduled folder or to create a new one. Tapping on the screen off the dialog box does not make it go away. Minimizing the app also fails to dismiss the dialog box.
What I've resorted to doing is choosing, "Select a scheduled folder", THEN I can X out/cancel the process resulting in no change.
The workflow needs to be streamlined here. Cancel and Undo options should be integrated.
Edit: If you don't want to use scheduled folders, I've found an easy, safe, and even useful workaround for this UI oversight: Go into Outlook Settings, Mail section, Swipe options. There change what swiping left and right does. I've changed mine to "mark read" and "flag". Neither is dangerous, and swiping has now become useful.