Well the poisoning theory sounds like the most plausible one...
They poison the mice/rats/other pests to get rid of them, without thinking about the consequences of what happens to "nature's clean up crew" the animals that eat other dead animals.
They eat the animals that have been poisoned, and consume the poison themselves. They die too...
It wouldn't be the first time.
Could be pesticide exposure, but... this sounds more likely.