Personally, I don't know all the details, as I've never written or maintained any addons that involve showing a drop-down unit menu, but there is a thread over on WowAce somewhere with all the details, and a way to work around it in some cases... though it was pretty old, so I'm not sure the workaround still works, and since none of the major unit frame addons implemented it, there was probably something wrong with it anyway.
