Rewrite rules disappearing

I’m working on a WordPress plugin which is utilising the WP_Rewrite class to add a custom rewrite rule. The rewrite rule is added during the “init” action and is activated upon plugin activation.

This works like a charm, but for some reason the rule disappears after a couple of hours. Visiting the “Permalinks” page in WP Admin fixes the problem for a couple of hours.

The URL should look like: site.com/contact/netherlands. “Contact” is a page with ID 49248, this page contains a shortcode which activates the plugin. The plugin should generate the appropriate HTML.

I hook into the “init” action this way:

add_action( 'init', array( $this, 'init' ), 10 );
add_action( 'init', array( $this, 'refreshRewriteRules' ), 20 );

Which calls this method:

public function refreshRewriteRules( )
{
    if ( get_option( 'resellers_flush_rewrite_rules_flag' ) )  {
        flush_rewrite_rules();
        delete_option( 'resellers_flush_rewrite_rules_flag' );
    }
}

public function init()
{
    add_rewrite_rule(
        'contact/([^/]*)$',
        'index.php?page_id=49248&c=$matches[1]',
        'top'
    );
}

Is there any way to detect why and when the rules are flushed?