What is causing the tablet to smoke when placing it on charger?

The tablet isn't what you should be worried about. Immediately stop using it and put it in a fireproof location, at least until you've confirmed it's not the battery at fault.

If not the battery, it's possible there's some short circuit somewhere in the device (e.g. on the logic board). If you're able to open up the device safely, it should be obvious where the catastrophic failure occurred. I would suggest first leaving it powered on, in a fireproof place, to gradually discharge the battery fully before poking at the device. Be prepared for it to spontaneously ignite at any time.