[bizerl (OP)]

What exactly happens in one's voice box to make a boy's voice go from high pitched, through a period of switching between high pitched and low pitched, then suddenly just low pitched.

It seems simpler to assume that as the larynx grew, there would be a gradual decrease in the fundamental frequency of one's voice. Why such a rapid transition?

[cheryl j]

I couldnt find a good answer to this, but people not going through puberty can have voice cracking when they are singing out of their range, or singing without warming up. My voice cracks when I shout. I also noticed when I lived alone and spent an entire day in my apartment by myself and not talking, it would crack when I answered the phone and started a conversation. Maybe as the male vocal folds are changing in size, the voice cracks even when only slightly strained, or perhaps one pitch is more comfortable or natural at one volume or speed of talking, but the old pitch is more natural at another. That would be my guess.