Possibly a transformerless design... regardless I would guess that the DC ground side is connected to AC neutral... (but only when it's plugged in the right way

Possibly a transformerless design... regardless I would guess that the DC ground side is connected to AC neutral... (but only when it's plugged in the right way and when your outlet is wired properly), so plugging it in backwards connects DC ground (the exposed shield on a USB connector) to AC Line voltage. ZAP!