By using those named GET variables such as year or monthnum you effectively short circuit the URL rewrite (if you are using pretty permalinks). This way you end up querying the main WP_Query instance first and effectively making yours completely useless (well, you do alter the posts_per_page attribute so there’s that). Anyway, the point is that there is a base class WP that issues a 404 when the main WP_Query instance doesn’t return posts (which is the case) and the 404 template is invoked (if your theme has it) or if it’s missing the index template is used (which seems to be confirmed by your question).
Now, the quickest fix is to basically rename your GET variables into q_year or q_monthnum for example.