This plugin uses get_users function and it doesn't have random order. You can check the orderby values here.
http://codex.wordpress.org/Function_Reference/get_users
You can filter query like this in your custom plugin or child theme functions.php.
add_filter( 'display_authors_widget_query', 'my_order_by_post_count' );
function my_order_by_post_count( $query_args ) {
$query_args['order_by'] = 'post_count';
return $query_args;
}
Maybe you could play with offset value like this.
add_filter( 'display_authors_widget_query', 'my_order_by_post_count' );
function my_order_by_post_count( $query_args ) {
$my_random = rand(0, 10);
$query_args['offset'] = $my_random;
return $query_args;
}