Most likely scenario:
The hot battery has one or more shorted cells and is presenting a low voltage to the charger thus accepting the lion's share of the charging current. Lots of current into a battery with shorted cell(s) will produce heat and lots of bubbling gas - hydrogen. Enough heat and the hydrogen can explode blowing off the covers. If the second battery is good and is connected in parallel with the hot battery while charging, it will also contribute greatly to the current flowing in the hot battery.
The second and less likely scenario is that the charger has failed and is applying a high voltage to the batteries. In this case the batteries would likely BOTH be warm from the continuous high charging current due to a high charging voltage.
You can tell if the hot battery is shorted by simply disconnecting it from the charger and waiting 30 minutes or so. Then measure the voltage of the battery with a digital voltmeter. If the voltage is under 12.6V, the battery is suspect. I would then wait overnight and measure again. If the voltage is around 10.5V or lower, it indicates a shorted cell. A shorted cell is non-fixable. The battery is toast.
