Battery is about to explode.

mikeculligan.mc this is becoming a more frequent situation. The bloating occurs when the batteries have been internally chemically degraded. The degradation creates gases that will expand and cause the bloating. For more on the aging process etc. you can download a good paper from here. So in short, the reason for the bloating is the age of the battery. Yes there are other reasons as well like overcharging etc, but those do not apply to your device.

Besides the why, you do want to remove the battery ASAP and discard of it properly. Since you did not tell us which tablet you have, we cannot assist you with that.