Panel not collapsing on page load.

I am working on an asp.net webforms application using VS2013.

I have two panels on a form. The first panel acts as intended, it is collapsed when the page loads. But the second panel is not collapsed when the page loads. Instead it collapses after I do anything else on the form like choosing a dropdown list or gridview - neither of which are located in the panel I am attempting to collapse.

The following is the code for the first panel that is operating correctly:

<asp:Button ID="btn_open_new_rvw_frm" runat="server" Text="Initiate A New Document Review" CssClass="frm_btn_style" Visible="false" /><br /><br />

<div class="row">
<asp:Panel ID="new_rvw_frm_panel" runat="server" style="overflow: hidden;">
<fieldset>
<div id="new_rvw_form" runat="server" style="background-color: rgba(128, 202, 255, 0.61);">
This is the content to be hidden or shown.
</div>
</fieldset>
</asp:Panel>
</div>

<ajaxToolkit:CollapsiblePanelExtender 
runat="server" 
BehaviorID="new_rvw_frm_panel_CollapsiblePanelExtender" 
TargetControlID="new_rvw_frm_panel" 
ID="new_rvw_frm_panel_CollapsiblePanelExtender" 
CollapseControlID="btn_open_new_rvw_frm" 
ExpandControlID="btn_open_new_rvw_frm" 
ExpandDirection="Vertical" 
ScrollContents="false" 
Collapsed="true" 
CollapsedSize="0">
</ajaxToolkit:CollapsiblePanelExtender>

There are a few gridviews located between the two panels.

Here is the code for the second panel which is not collapsing on page load:

<asp:Button ID="btn_open_new_sub_rvw_frm" runat="server" Text="Submit A Document For Review" CssClass="frm_btn_style" Visible="false" /><br /><br /> 
<asp:Panel ID="new_rvw_sub_frm_panel" runat="server" style="overflow: hidden;">
<fieldset>
<div id="div_add_rvw_sub" runat="server" style="background-color: rgba(128, 202, 255, 0.61);">
This is more content that needs to be hidden or shown.
</div>
</fieldset>
</asp:Panel> 
</div> 

<ajaxToolkit:CollapsiblePanelExtender 
runat="server" 
AutoCollapse="false" 
AutoExpand="false" 
BehaviorID="new_rvw_sub_frm_panel_CollapsiblePanelExtender" 
TargetControlID="new_rvw_sub_frm_panel" 
ID="new_rvw_sub_frm_panel_CollapsiblePanelExtender" 
CollapseControlID="btn_open_new_sub_rvw_frm" 
ExpandControlID="btn_open_new_sub_rvw_frm" 
ExpandDirection="Vertical" 
ScrollContents="false" 
Collapsed="True" 
CollapsedSize="0">
</ajaxToolkit:CollapsiblePanelExtender>

What am I doing incorrectly?

Thanks,

J