> Before my ill-fated rip cut, the board looked straight and true, with 
> no
> indication of internal stress.

We used to call that phenomenon "case hasdening."  The exterior of the 
wood
is straight and true, but a little deeper into the piece the stress is
tight.

> Serves me right for using a tailed apprentice!  Still, it is a poor 
> workman
> who blames his tools . . .

If it's any consolation, the same thing would have happened if you hand 
ripped
it.  It wasn't you.  It wasn't the tailed apprentice. It was the tension 
inside
the wood.  I've had it happen numerous times.  Alder, pine, redwood have 
all
done this for me.  The most exciting time happened when one of my 
partners was
running a four inch thick by 26 inch wide piece of cottonwood through a 
tailed
apprentice planer.  From deep inside the machine, we heard a loud boom 
when
the inside of the timber was released from its bondage.

Mike in Sacto