Why is the formation of slacked lime accompanied by a hissing sound and the formation of bubbles?
Good eye for the inconsistency.  Cultivate that, it will serve you well.  But this is an apparent, rather than a real inconsistency.  The reaction is VERY exothermic.  Check out how they make quicklime to find out how much heat it takes.  

Where the water touches the quicklime, enough heat is evolved to boil a little bit of the water.  Hence the bubbles and hiss.