I had a similar (intermittent) problem with the RadDropDownButton freezing the UI (until the mouse was moved outside the bounds of the window).
It appears that within the RadDropDownButton it uses an AutoClosePopupWrapper which has code like this (decompiled):
And, of course a 'Run' element is not derived from UIElement (but is still a DependencyObject and a FrameworkContentElement).
In my case the problem was because I was using a ListBox inside the dropdown content. Looking at the events being fired from the RadDropDownButton, specifically the IsMouseCapturedChanged, GotMouseCapture and LostMouseCapture events, I could see that the times when the UI froze up was when a LostMouseCapture failed to fire (e.g. the dropdown that closed was still holding on to the mouse capture).
I am attempting to fix this by replacing the ListBox with a RadListBox and so far it seems to be working. But this is an intermittent bug so not easy to be sure it is fixed.