add page_Load event as a trigger of update pane
Have you read on what triggers do and how update panel works? Have a quick read here.
Update Panel is for partial rendering. In case you need to execute a certain method you can always write about that in the triggered control event (and the UI related must be inside update panel)
You need to make sure about your page design. Having a scenario if any to trigger a Pageload does not make any sense.