To scroll to the top every time in all the browser apply the following solution:

OnClientDropDownOpened="onOpened"

The OnClientDropDownOpened event handler: 

function onOpened(sender, args) {
    var ddl = sender.get_dropDownElement();
    if (ddl) {
        var RadAutoCompleteBoxPopup = ddl.children[0];
        var racList = RadAutoCompleteBoxPopup.children[0];
        racList.scrollTop = 0;
    }
}