Index: wc-post-types.php
===================================================================
--- wc-post-types.php	(revision 941)
+++ wc-post-types.php	(working copy)
@@ -171,7 +171,7 @@
 		$current_screen = get_current_screen();

 		// Order by session time
-		if ( 'edit-wcb_session' == $current_screen->id && $query->get( 'orderby' ) == '_wcpt_session_time' ) {
+		if ( 'edit-wcb_session' == $current_screen->id && ( $query->get( 'orderby' ) == '_wcpt_session_time' || $query->get( 'orderby' ) == '' ) ){

 			// Voodoo because of http://core.trac.wordpress.org/ticket/23268
 			$query->set( 'meta_query', array(
@@ -187,6 +187,9 @@
 					'compare' => 'NOT EXISTS',
 				),
 			) );
+			if( $query->get( 'orderby' ) == '' ){
+				$query->set( 'order', 'asc' );
+			}
 			$query->set( 'meta_key', '_wcpt_session_time' );
 			$query->set( 'orderby', 'meta_value_num' );
 		}