The default DS creatures are really prone to becoming senile a lifestage or two too early (though oddly, this seems to be solely a DS problem - I've never seen it happen in C3 standalone).
The reason for this has something to do with the combination lobe, as Vampess explains:
Vampess wrote:
The combination lobe, and more specifically the drive tracts leading to it, store the learned knowledge of how to reduce drives. When creatures learned something, the previous value was overwritten, which caused the creatures to seem very forgetful. Because the learning is slightly random, some creatures would even refuse to perform a certain action, as it was perceived as a bad action.
I don't think there's much you can do to help your current creatures, but in the future you could give the Creatures Full of Edits a try. While this could still happen with them, it's much less frequent. Assuming you're not already using the CFE, which in that case you've probably just run afoul of some bad luck.
To answer the cannibalism question: no, creatures cannot eat each other by default. Sometimes they just randomly spit out what's on their minds even if that's not really what they intended on doing.