I’ve found – and I know others have too – that you can’t delete tags in WordPress from the tags management screen.
However, after recently upgrading my WordPress version I found that, after leaving my plugins disabled by accident, it would work. Enabling my plugins then broke the functionality. Unfortunately, my good fortune didn’t go as far as thinking to test it and try and work out which plugin is the cause.
So, yes, that’s the cause. But exactly what I’m not sure.
Next time I have some tags to delete and some times on my hand, I’ll try and work it out further.