UILayout.prop for enum type property by default expands up, which is different from UILayout.menu which by default expands down.
This behavior creates inconsistency when menu type dropdown used along with enum prop type dropdown in the UI.
Here is GIF animation to illustrate the issue:
I think I have never seen dropdown list that drops up by default in any other app before. And whatever was the reason to make enum prop list expand up instead of down by default, it is not good enough to break the primary behavior of dropdown lists.
This is not the add-on issue, it's the general behaviour of UILayout.prop for enum type property, which is often used in Blender UI: all "dropdown" lists in 3D View header, Object Draw Type "dropdown" etc.