ok..If you carefully look at the vi you will come to know the answer for your question.
The reason is because when the value of the slider becomes greater than 8 the pop up will appear and when you close, it will close (for say i=1). Now again the pop up will appear and close when you hit the close button. (say for i=2).
This operation will continue unless and until some one will change the value of the slider to be less than 8. or the vi is aorted
Also do not use the contineous run button unless and until you are debugging something. Instead use a while loop.
A better approach would be to use a event structure wired with a valus change event for the slider.
Regards
Guru (CLA)