SOLUTION FOUND IN AN OTHER DISCUSSION :
Well, this is a known issue when you are using ButtonType of “Image” for the commandField to post the page twice.
<asp:CommandField ButtonType="Image" DeleteImageUrl="http://tango.freedesktop.org/static/cvs/tango-icon-theme/16x16/places/user-trash.png"
Using ButtonType="Link" will solve your problem.
Well, the reason for this is, of the three button types (Button, Image, Link) for the CommandField, the first two (Button and Image) are of “input” type (Button input, type = submit) , (Image input, type=submit) and the Link is anchor tag.
My workaround for this problem is to use a TemplateField
<asp:ImageButton id="DeleteButton" CommandName="Delete" ImageUrl="~/images/delete.ico" runat="server" />