It's not the weight of the bricks, nor a chain reaction during a "legitimate" (real brick) demo, but the force generated by the striker, traveling down through the stack.
If the strike doesn't continue to accelerate throughout the process, the breaking will stop at the last brick to be accelerated through (and maybe break the strikers hand, too, if he slows down and hits the rest of the bricks with less-than-adequated power).
Been there, done that.
But I got better.
That was many, many moons ago in my foolish youth.
If the striker uses spacers between each brick, then he is breaking only one brick at a time; with no spacers, his acceleration is penetrating the stacked bricks almost simultaneously.
The whole breaking demo thing at its worst is a laughable sideshow trick with all the silly crap -- flaming lighter fluid, fancy costume, drummers. At best, it is a simple way for a person to test the elements of his punching/striking form -- proper bone alignment, proper power generation and proper acceleration throughout. The fist must continue to accelerate throughout the entire strike, or you get disaster.