becoz the parent /master page object has to be finished own task before the child page.

http://stackoverflow.com/questions/20040985/why-unload-event-of-masterpage-calls-before-unload-of-aspx-pagecheck this link for answer

Hi, I was trying to find the sequence of events between Master page and aspx page, then I came to know that Unload event of masterpage calls before calling of unload event of aspx page.