help with redrawing -

I have a control in an update panel, its a listbox.  On SelectedIndexChanged of the listbox I have a simple routine to change the backcolor of some of the items in the box (see code attached).  For some reason, this code redraws the whole page, annoying the users.  Interesting though that this doesn't happen when I am debugging locally, nor does it happen in firefox - only in IE on the server.

Any ideas what could be happening?

 Protected Sub lbUp_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles lbUp.SelectedIndexChanged
        ChangeBackColorIfImplied()

    End Sub

 Protected Sub ChangeBackColorIfImplied()
        Dim SelectedNum As Integer = -1
        Dim SelectedString As String = ""
        Dim StringofUps As String = ""
        lblUpInfo.Text = ""
        Dim i As Integer
        For i = 1 To lbUp.Items.Count - 1
            If lbUp.Items(i).Selected Then
                StringofUps = StringofUps & lbUp.Items(i).Value
                lbUp.Items(i).Attributes.Add("style", "background-color#FFFFFF")
            End If
        Next i
        Dim Flag As Boolean = False
        Dim mylistitem As ListItem
        For Each mylistitem In lbUp.Items
            Dim ListItemString As String = mylistitem.Value
            If InStr(StringofUps, ListItemString) > 0 Then
                If StringofUps <> ListItemString Then
                    mylistitem.Attributes.Add("style", "background-color:#99FFFF")
                    lblUpInfo.Text = "All entities and people highlighted in light blue will also receive this communication"
                End If

            Else
                mylistitem.Attributes.Add("style", "background-color:#FFFFFF")
            End If
        Next
    End Sub