MEATSts spawn at random on the ground, but they are also generated by players who use their movement burst. MEATSts will also be generated when a snake dies. The MEATSts will spawn in clusters of around the same size the snake's body segments had when it died.
There are also special MEATSts that appear larger than most other MEATSts and move around the screen, generally trying to avoid the player. When eaten, these special MEATSts give a much larger amount of length to the player and so it can be worth it to use your speed ability to grab them, especially early on in a game.