Monsters are triggered to attack when a Prisoner MOVES within their line of sight, by FALLING onto one, and/or in a CHAIN REACTION, where another Monster’s attack hits them.
While movement in their line of sight triggers an attack, moving out of a Monster’s line of attack evades their strike and its penalties.
This is true of Falling through a Crumbling tile or Leaping into a Pit, as well. It’s like taking a side Passage. The Monster will be triggered, but Falling avoids the attack.
Falling onto a monster triggers the Monster immediately, attacking the Prisoner who landed on it AND any Prisoner in line of sight. After the attack, the falling Prisoner scrambles to an adjacent space, either an existing tile, or a newly drawn one.
If a Wax Eater’s attack hits another Monster, that Monster also attacks in a chain reaction. Below, a single move by any Prisoner will trigger all Monsters. Only the first player to Move or Fall can avoid being hit.