The issue is Radio Buttons Are Not Mutually Exclusive When Used in a Repeater Server Control.
From Microsoft
This is a known issue of ASP.NET template databound control(such as repeater, DataList …). When you put RadioButton in such control's template, since its "groupName" is assigned a unique value each row,. Here is the public kb article document this issue:
Server Control
http://support.microsoft.com/kb/316495/en-us