Well, Saudi Arabia is dropping our bonds because they're quickly going broke. They're actually issuing their own bonds this year or early next if they're not already on offer. It's a huge offering, too.
China's doing it because they're also really unstable right now. Their currency has been dropping and they're trying to stabalize it against the dollar.
This is definitely something to keep an eye on, but is not yet what I'd call a red flag. Maybe a slightly pink-tinged flag, but nothing to panic over.