Important: The issue only occurs when the Buttons are spread around in different rows in RadGrid. 
When multiple RadButtons (ButtonType="ToggleButton" ToggleType="Radio") are grouped through GroupName property, the server-side property checked doesn't take effect when the button which is to be checked is declared before the current checked button.

The workaround is to execute JavaScript from the server which sets the checked property. For example:

C#:

        string script = "function f(){$find(\"" + RadButton1.ClientID + "\").set_checked(true); Sys.Application.remove_load(f);}Sys.Application.add_load(f);";
        ScriptManager.RegisterStartupScript(this, this.GetType(), "key", script, true);