Hello Justin,
Lucky guess maybe? ;)
Seriously, I am not familiar with your exact setup and if you execute other code in OnClientClose, but I assumed that the RadWindow is modal - sometimes it happens so that the modal layer is still present on the page while code in OnClientClose is executed and when the layer is removed, the browser automatically returns the focus to the parent page. There are couple of other scenarios where a problem with focus might occur (e.g. opening a modal RadWindow from within another RadWindow by clicking on an imagebutton), but I decided to suggest the timeout first :D
Best wishes,
Georgi Tunev
the Telerik team