Good question, OJA!
Hm... By the looks of it, Sligs
did shut them in the pens, because if they didn't, they probably wouldn't be patrolling them.
The Slogs are angry at the Sligs for doing this, and when Munch drops the Sligs in, the Slogs attack.
Or perhaps the Slogs live on little food, and when the Sligs are dropped in, they get so hungry that they eat them.