Is your sig correct? If so, isn't a 60 cube a bit small for tangs? That only gives max 2 feet of swimming room.
Now that I'm done being the tang police, from what I've seen/experienced, a BTA doesn't generally eat fish unless the fish is quite sick to begin with. How long did you have the tang for? It's possible the fish was weak from stress, parasites, or other internal infection and you had no idea about it. It was then too weak to escape the BTA. I would argue that about 99% of healthy fish should be able to avoid/escape a BTA's sting if one or even a few tentacles brush against it.