What causes blocks to cascade in Tetris Effect: Connected?

In Tetris Effect's new multiplayer mode, Connected, 3 players work together to take down an AI-controlled Tetris board. After the players clear enough lines, they enter the Zone, where all existing blocks cascade down, and players combine across all 3 boards to clear lines. During this Zone section, there are times when blocks will continue to cascade, and I cannot figure out why. It is not immediate (ie. you drop a tetromino in a bad spot, with open space below it, the blocks do not cascade), and I can't accurately reproduce the cascade action.

So, what causes blocks to cascade at times during the Zone in the Connected multiplayer mode?

TL;DR - Special purple pieces referred to as "Magicminos" that appear at random.

According to the wiki, cascading gravity is enabled in the Zone by utilizing special purple pieces:

On the team's side, special purple pieces, which appear at random but more frequently in early phases of the game, can be used to eliminate gaps by cascading blocks down the stack.

These purple pieces are typically referred to as "Magicminos" (in some circles), and they are shaped like typical Tetrominos, just with a bright purple hue:

Furthermore, as you pointed out, when you first enter the Zone, an initial cascade is triggered to eliminate all existing holes in the board and enable players to work together more effectively.

You can see the "Magicmino" in action in this YouTube video which starts where they're talking about "Magicmino" and cascading specifically: