Well, fact is more important than my dumb hypothesis! So watch this video:
How larynx produces sound
At high pitch we may fail to hold the cords closer together possibly due to lack of power in muscles of larynx, the voices cracks. Cracking is nothing but cords failing to open and close smoothly start jerking causing sudden discontinuity in frequency introducing noise. I think.
Here is an explanation how sound is produced.
Watch this as well! You lazy webpage watchers, read them Internet is not TV and webpages are not for watching read them :)