I've also been experiencing this issue, my prisoners have been breaking free as they are led to be pitted. Has anyone found the reason behind this?
Edit:
Prisoner can escape while being "hauled" for pitting, even if that distance is a single tile. Critters climbing out of the pit is permitted by the hatch not being locked (or possibly the pitted creature being a gremlin or kobold), but that's a completely different issue.
Not only can creatures designated for pitting escape, but it seems of one escapes all the subsequent one do as well, even if the first one has been put down when the hauler for the next one arrives. I've given up on mass pitting for that reason.
It seems that the first prisoner to be pitted usually gets pitted fine, but all the the subsequent ones break free. My guess is that this might be due to the first prisoner setting some sort of "combat flag" as soon as it's released into the pit, which makes the other prisoners enter combat as well, but I'm not sure.