If your charger is a really cheap one its open circuit voltage could be in the order of 24 volts or so which is adequate to tickle you. Alternatively there may (unlikely though) be a smoothing inductor in the DC line which is providing a back-emf kick when you break the charge current.
But to get a tickle you need a return path, so it begs the question "what part of your body was in contact with the other pole of the battery or charger". If none, then put the charger in a place where it may never be used again !!