Page#onInitialize is called when the constructor of Page throws an exception, i.e. when the Page is not initialized correctly. This can cause additional exceptions which are usually added to an error log even in cases in which the exception in the constructor is handled (by Wicket). This issue is possibly related to
Another case of the problem occurs when setResponsePage(...) is used in the constructor to navigate away from a page that can't be initialized correctly.
I'm attaching a quickstart.