There is a limit on how many concurrent requests you can send out. Also see here: How many concurrent AJAX (XmlHttpRequest) requests are allowed in popular browsers?
One trick websites use to make their serving faster, is to limit the amount of requests needed. They also put stuff (images) in a a separate (sub)domain, because you get another 3 (or 6) requests on that domain.
In your case you can try to fit them into 1 call, or direct them to several subdomains.
(to be clear: although the server CAN have a limit, the limit you are reaching here is the requests your browser sends out. in IE it seems you can be limited to another (2?) number, see the link I provided)