Its rare but it happens. It has to do with the way they sleeve the blocks.
The old way they would cast the block and bore it out for an interference press-fit steel sleeve. This is ok but as the block heats up the aluminum expands at about twice the rate of the steel so the sleeves would get a little loose.
The way they make these newer HO v6's is that the heat the sleeves up and pour the cast around the sleeve that way when it up to temp the sleeve is still pretty tight to the alum casting...but when it cools down the alum can crack since the alum shrinks twice as fast as the steel sleeve.
...If happens every once and a while, but not very often, but this is a prime example of why they put warranties on new cars.