UpdatePanels give basic AJAX functionality, but they do have some idiosyncrasies.
First, always remember to set the UpdateMode to Conditional, otherwise whenever the page is posted back for any reason, the update panel will postback as well.
If using a timer, it must be inside the ContentTemplate, and should be set as a trigger for the panel, to make it obvious. Don't want to have a bunch of mysterious postbacks to hunt down.
A final point: If you have more than one UpdatePanel, remember to do this for each one. One rogue Timer on a page can cause chaos through unexpected postbacks.