RadTabstrip always reloads all of its pageviews (via the PageViewCreated event of the multipage control). This means the code-behind of your page / UserControls will be fired no matter which pageview is rendered. Loading all pageviews is essential for the server controls living in the page views. If we don't load them every time viewstate will be corrupted and postback events won't fire.
All the best,
the Telerik team