WPF Frame control appears automatically!

I have a frame on my main Window.xaml, when I click a link on the main Window, a Page.xaml page should appear. Only when I click the link, the page should load, but the page like auto runs without even displaying the main window. The main Window disappears in a flash and the Page.xaml appears. What should I do??

Between, I use MS Expression Blend & C# for the development of my application.