In ASP.NET, it is common to have some data members of our Page class (one who derived from System.Web.UI.Page). It is important to keep in mind that those members are re-defined or re-declared every time a new instance of our Page class is created. This happens each time there is a round-trip between the server and the browser (i.e. but may not limited to: Page_Load event). Therefore, we need to be aware of the state of those data members.
One more thing: even if an UpdatePanel is used in a page, Page_Load event still gets fired every time.