Why is the app waiting for the debugger when its not connected to computer?

Restarting is more time taking, Easier way is that in the device, select "Developer Options" > “Select debug app” and select "Nothing".

For me, the solution is to select "None" in "Developer Options"->"Debug"->"Choose debug application", though it already has "None" selected. Seems like the device put a "need to debug" label on my app sometime before which is still there when I "Run" the app on the device using my IDE (or even launch the app manually when the device is not connected to PC), and re-select "None" removes the label. Don't know whether it's the case.

Just to close this question: I restarted the device, and that helped.