It happens when you have a long list of drives and there's not enough space to drop down the list box. According to the listbox's default behavior, if there is enough space upwards the listbox will actually drop "up", so to speak.
The description list, however (which is a separate window) is still shown below the listbox, so we get a kind of "broken" look of it.
Image: http://img27.exs.cx/img27/3905/tcdriveslist1kp.png
