This is because as the code is originally written, the page title is not re-populated on postbacks.
Easily fixable by moving the following two lines in Page_Load to appear BEFORE the if(!this.IsPostBack), instead of inside it.
Code:
// try to load the article with the specified ID, and raise an exception
// if it doesn't exist
Article article = Article.GetArticleByID(_articleID);
if (article == null)
throw new ApplicationException("No article was found for the specified ID.");
this.Title = string.Format(this.Title, article.Title);
if (!this.IsPostBack)
{
... etc ...
Page title reads Article:{0}
