I suppose this is a bug in RadComboBox. I will explain a little bit more. When FilteringMode == Contains or StartsWith, RadComboBox searches through its items for items that match its Text. Those items that match are made visible, the rest are collapsed. In order to display all collapsed items when the popup is opened, RadComboBox runs such matching cycle with empty text every time the dropdown is opened. This is the expected behavior (in my opinion) when FilteringMode is not None, but when it is None, the container bindings should work and this is the case I am going to fix. I would be glad to hear your opinion about that behavior.
Unfortunately it is too late to include the fix in the today's internal build, if this problem is not a showstopper for you, I would recommend waiting for the next internal build, next friday. Otherwise, I could provide you with a build earlier next week.
Other possible (and probably better) solution would be to instead of hiding the items that should not be visible, to remove them from the ItemsSource of RadComboBox.
the Telerik team