The tooltips were turned off on the debuffs displayed next to the HUD because they were interfering with people clicking in the field of play during combat.
If you want to turn them back on, use a plain text editor like NOTEPAD to edit the two files
[ Interface > AddOns > nUI > Layouts > Default > HUDLayouts > {hud mode} > nUI_HUDSkin_{hud mode}_Player.lua ]
and
[ Interface > AddOns > nUI > Layouts > Default > HUDLayouts > {hud mode} > nUI_HUDSkin_{hud mode}_Target.lua ]
note: {hud mode} is one of PlayerTarget, HealthPower, etc. depending on which HUD you use.
Do a search for the
["Aura"] section in the file. The first block of auras in the file,
[1], where
aura_type = "harm" is the one you need to change. Look about half way down in the options section and change
clickable = false to
clickable = true and save the file.
That should make the tooltips works again. Just note that clicking anywhere in the field where auras are hidden when not displayed will interfere with the mouse (which is why I have that turned off at present).