WP_Query not getting all posts, just tagged posts

You’ve given WP_Query and empty argument set, effectively asking it to get “nothing”. Try:

$args = array ();
$query = new WP_Query( $args );
var_dump($query);

As soon as you give it something to latch onto, like your tag argument, the query works. To get all of your posts pass a post_type argument.

$args = array ('post_type'=>'post');
$query = new wp_query( $args );
var_dump($query);

Or to get all post types…

$args = array ('post_type'=>'any');
$query = new wp_query( $args );
var_dump($query);

Or to only get particular post types…

$args = array ('post_type'=> array('post','book'));
$query = new wp_query( $args );
var_dump($query);