a4j:commandButton causes full page reload on IE7

Our process allows users to activate their account, and then configure e-mail preferences. We're using the tag:

<a4j:commandButton id="activate"
                   action="#{controller.agreeAction}"
                   image="/img/ok.png"
                   styleClass="activate-button"
                   reRender="mainContent, sideBar"
                   oncomplete="showEmailDialog();" />

This works fine on Firefo开发者_JAVA百科x, but on IE, the showEmaiDialog() fires off to display the new dialog, and then the full page reloads, which instantly hides it again. I put in numerous alert() calls to make sure of what was happening. I see the e-mail dialog until I clear the final alert box in in the showEmailDialog() script, and then I see the alerts that I put into jQuery(document).ready(). Why does IE do a full page reload instead of just refreshing the requested sections?