Arcing to a drywall nail... why?

I have this light fixture that started smoking and popping today, so I turned the power off to the breakers and took the fixture down. This is what I saw (2 pictures).

It appears that a bare wire on the light housing was arcing against a nail in the drywall??

It's an old fixture, replacing it shouldn't be a problem for me, but I just have to ask somebody - is the drywall nail likely to have been driven into a wire or something? Why would the fixture arc to it like this? Presumably a normal nail is just going into a piece of wood, no path to ground?

But I'm over my head really, so I'm asking you. Should I have an electrician check this out?