They steam because there is a large amount of hot rock present and when it combines with the high volume of ground water in the crater, the heat causes the water to boil – think a boiling kettle.
There are also volcanic gases mainly CO2 and some sulphides (these give off the unpleasant smells).
The steaming is most prominent at the top of the new dome and at the interface between the old dome and the new because there are now more gaps through which the steam can come through.
The fumarolic activity along the old dome will also steam more due to the increased activity within the crater.