First, search for a function called doPostback and set a breakpoint at the entry point and click the button. If you hit this breakpoint, it could mean that autorepeat is enabled for the control generating the button. However, if you run this breakpoint, you should be able to view the stack trace to figure out how you got there.
If this is not found and you have access, download one of the master pages from http://startermasterpages.codeplex.com/ and temporarily replace your master page with one of them. Take a screenshot of the scripts loaded on your page. Then add them to the starter home page one at a time until the unwanted behavior appears. Then set a breakpoint on each function entry point in the script and click the button and watch where you land.