Physics.

When the boat flips, it typically stops going over at the halfway point. The sail is on the surface and out of the wind so there's no more force pushing it over. People who were hanging over the side are all dumped in the water so they're not pulling the boat over either. The masthead is in the water and floating, producing a righting force that fights any tendency for the flip to continue. The mast partner is rolling against the mast, pinning it in place.

In 40 years of sailing I've never had an unattached mast come out of a boat unless I deliberately took it out.

Laszlo

   This is helpful.  Thanks for confirmation.  I was just not 100% sure that I had rigging correct.

And for a real world example of this principle - Capsized Jimmy skiff

 Any suggestions for getting removing the mast after capsize? I capsized the other day and could not get the mast out because of the tension of the mast floating. A friend with a pontoon helped me pull on the gunwale and it took the tension off, but damaged the deck.