Ok, I see.
It was base down, and it wasn't zip cord, it was SVT, as it is always used here for lamps etc. Why not have that dangle down? There was a strain relief on the socket! It was a modern metal socket (they still sell them here, brass or iron) with porcelaine guts. The socket didn't fail, only the cord insulation. After some time the wires got contact and shorted.
I've seen several plastic sockets fall into small pieces due to using 100W bulbs in them.
And as I said, the faults stopped after we replaced the cord instead of just shortening it.
