Well it was a slight assumption - but a common one when redirecting key combinations to another action.
It basically cancels the UI action that raised the event. So in this case, since Ctrl X would normally cause a Cut, it cancels that action and performes the code based alternative only.
It occurs to me that you might have wanted this functionality to both cut and automatically set focus to one given control into which you would be pasting the cut text. That's fair enough - but there are alternatives to "moving" text between controls if you so chose.
Just remove that line if you still want the standard Windows Cut operation to occur.
Cheers.