Drop Down Firing Wrong Event?

I'm using VS 2010, .net 4, vb.net. I have an asp.net site and on a page have 2 drop down lists.

<asp:DropDownList ID="ddlClassIndex" runat="server" AutoPostBack="true" />
<asp:DropDownList ID="ddlClassQBankList" runat="server" AutoPostBack="true" />

When the page first loads i can click on either of these, select an item and the event fires correctly. When i select either of the drop down boxes as a second operation, the first drop down, ddlClassIndex.SelectedIndexChanged fires. What have i done wrong that this would be happening. I do a page.ispostback=false check when the page loads and we are also using master pages.

Private Sub LoadClass()
Dim Os As List(Of Dempsey.OldeTestClassIndex) = Nothing
Dim Oc As New Dempsey.OldeTestClassIndex_Controller
Os = Oc._GetClassIndexData(0)
Dim X = From w In Os
Where w.Active = 3
Select w
Os = X.ToList
With Me.ddlClassIndex
.DataSource = Os.OrderBy(Function(o) o.ClassTitle)
.DataTextField = "ClassTitle"
.DataValueField = "tblClassIndexID"
.DataBind()
.Items.Insert(0, New ListItem("Choose Class Title", "0"))
.SelectedIndex = 0
End With
End Sub

Protected Sub ddlClassIndex_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddlClassIndex.SelectedIndexChanged
If Me.ddlClassIndex.SelectedIndex > 0 Then
If ClassHasCategories(Me.ddlClassIndex) = True Then
Response.Redirect("AddEditQBank.aspx?ClassIndexID=" & Me.ddlClassIndex.SelectedItem.Value)
Else
Me.lblStatus.Text = "No Categories"
Me.lblStatus.Visible = True
End If
End If
End Sub

Private Sub LoadClassQBankList()
Dim ary As New ArrayList
ary.Add(1)
ary.Add(2)
ary.Add(3)
Dim Os As List(Of Dempsey.OldeTestClassIndex) = Nothing
Dim Oc As New Dempsey.OldeTestClassIndex_Controller
Os = Oc._GetClassIndexData(0)
Dim X = From w In Os Join l In ary On w.Active Equals l
Where w.KT = 1 And w.RandomTest = 1
Select w
Os = X.ToList
With Me.ddlClassQBankList
.DataSource = Os.OrderBy(Function(o) o.ClassTitle)
.DataTextField = "ClassTitle"
.DataValueField = "tblClassIndexID"
.DataBind()
.Items.Insert(0, New ListItem("Choose Class Title", "0"))
.SelectedIndex = 0
End With
End Sub
Protected Sub ddlClassQBankList_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddlClassQBankList.SelectedIndexChanged
If Me.ddlClassQBankList.SelectedIndex > 0 Then
Response.Redirect("ClassQBankList.aspx" & "?ClassIndexID=" & Me.ddlClassQBankList.SelectedValue)
End If
End Sub

~~~~~~~~~~

More info
I am checking for page postback. When i select from the drop down, it redirects to a different page. When i click the back button and selected another value in the other drop down, both SelectedIndexChanged fire. So i do i need to do something with viewstate? before i redired to the other page?