In Mob File

  Skills:
  - skill{s=YetiScream} @trigger ~onChangeTarget 1

This should trigger it when it changes target with a 100% chance to activate, and target whoever triggered it

Skill File;

YetiScream:
  Cooldown: 1
  Skills:
  - effect:explosion @Self
  - sound{s=entity.endermen.scream;v=20} @PlayersInRadius{r=30}
  - damage{amount=5} @PlayersInRadius{r=10}
  - throw{velocity=25;velocityY=8} @PlayersInRadius{r=10}