The RadAjaxManager uses the Array.enqueue()
If you are experiencing cases whereby a request is either processed on the server or returned to the client out of order, remember that when the requests are transferred over the intranet/internet they do not always take the same route and are not guaranteed to arrive in the order in which they were transferred. It is usually the case that messages (or in this case, requests) sent across a network will not arrive in the order in which they were sent due to network latency, routing, and other issues.
I hope this helped clear up your concern. If you'd like me to elaborate further or answer any additional questions you may have, please let me know.