Your expected behavior is not exactly how master pages work. There may be ways to achieve a no postback solution in this scenario but the easiest one would be to use an <IFrame /> (which is usually frowned upon)
explanation:
ASP.NET engine takes your aspx and puts it inside the master page and then renders it as one page, then serves it to user.
If this is not what you want and you want that only content of your master page be loaded, then you should not use master pages at all! It's against nature of master pages. they act like skins for aspx pages.
Search for HTML IFrame tag and don't use master page.
P.S: IFrames are not used widely in this days.
Method 2:
In order to Avoid Master Page Postback i.e only to refresh the content within ContentPlaceholder, we need to do is add following two lines in the <head> tag of the MasterPage.
<meta http-equiv ="Page-Enter" content ="blendTrans(Duration=0)"/>
<meta http-equiv="Page-Exit"content="blendTrans(Duration=0)"/>
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