Just a little bit further information on this. A USB device is limited to how much current it can take until it has been 'granted' more. It's something very small like 15ma, then it must get it's USB endpoint working and request the electricity.
It sounds like the battery is just shorting, and taking as much power as it can from the USB rail. As for the protection on the data pins, there normally isn't much in a shield, there will be a pull up resistor but you won't find an optoisolator, so 'leaking' power down that is possible, but it's a much harder manufacturing flaw to make than simply shorting in the battery. I doubt very much that the connector would have the data pins connected in an ultra low price manufacturing point.