That's an excellent question! I've often wondered myself. My guess is that they are rebuilding the Intellisense information used to display the Class View itself. Since the data can change, they probably clear out the items in the view so they don't have to implement a huge solution to keep track of updating the displayed data. For those of you who think the Class View should keep your selected node displayed when building, you can always send your feature request to email@example.com.
This was first published in April 2004