You must see that you have actually have a server side event for this
<asp:button id="Button2" runat="server" text="Button" onclick="Button2_Click" onclientclick="javascript:return isStatValid(this)" xmlns:asp="#unknown" />
This button has 2 events one client side and one server side
First it executes client side event and if that returns true it will execute server side code
protected void Button2_Click(object sender, EventArgs e)
{
}
This is the Event Method you will find on CS code page.
In your case there is no server event for the button
onclick="Button2_Click"