Details View control is a data-bound control that renders a single record at a time. event is raised when an Update button within the Details View control is clicked, but after the update operation.Detail View is one of the important control, that is mainly used in master-detail scenario.You select the record from master control (as example Grid View) and selected record is displayed in Detail View control.
This page is a great reference for Form View controller: View Update: I've updated code to refelct Phaedrus suggestion. Try removing this line of code and let me know if it resolved your issue.
protected void Detail View1_Mode Changing(Object sender, Details View Mode Event Args e) protected void Detail View1_Item Updating(Object sender, Details View Update Event Args e) protected void Details View1_Item Updated(object sender, Details View Updated Event Args e) The Mode Changing event is raised when a Details View control attempts to change between edit, insert, and read-only mode, but before the Current Mode property is updated.