Originally Iris Martindale had a much more humorous demise scripted. Iris was suppose to be lured into a quicksand pit by butterflies, where she was to drown. The scene was shot, but it was deemed too silly for the final film. The scene was re-shot to have Iris killed by a rattlesnake instead. Clips of the original death scene can be seen in the films trailer.
Because of the film's poster art, which depicts a human hand hanging out of a huge frogs mouth, it is often misconstrued that the film is about giant man-eating frogs rather than about swamp wildlife on the rampage.
Although it's implied that most animals have turned on Man, surprisingly, Crockett's family dog stays loyal until the end, which may imply that dogs are not part of the animal revolution. This would support the popular saying that "dogs are Man's best friend".
At first, the animals seem to have a clear motive for their violent and often premeditated actions - they are attacking the Crockett's out of revenge. The grandmother is a butterfly collector, the grandfather has a trophy room, the others are complaining about constant croaking and want the frogs poisoned and dead out of convenience, etc. The animals even seem to spare Sam Elliott's ecologically-aware character in the beginning. However, the animals eventually attack even him, although, in theory, that might be because he decided to help "the enemy" - the Crocketts. Of course, if the "animal apocalypse" theory is correct, then Elliott's actions don't influence their deadly behavior, after all.