I was wondering if any person has experienced this situation. From time to time, when I cast Insect Swarm, enemies polymorph into rats.

I don't recall that being mentioned before.
Does it happen more with certain enemies, etc? Maybe I'll re-install BD this weekend and try out Insect Swarm.

It's only been happening in the Battlefields for now with Black Ring Clerics and guards. This all seems random. I don't see it happening anywhere else for now.

I loaded a save in the act 4 battlefield, with edited characters for extra skill points and stat points (so I could ignore mana requirements), and was able to replicate this. Maybe it was just a coincidence, but for the time I played Polymorph seemed to be more likely with any groups where there was a Black Ring Mage that used Disease (the area effect with green jets of gas coming out of the ground).

Seems like an unintended side effect of the spell, Insect Swarm.