Stomach irritation, vomiting, diarrhea are all caused by multiple things such as: poinsettias, chocolate, coffee, onions/garlic/chives.
Liver/kidney failure is from: chcoclate, coffee/caffeine, and xylitol.
Accidental exposure is probably the most likely. Or being unaware of what animals cannot eat.
This can be reduced by having proper knowledge on toxic things to pets and by making sure toxic items are not within reach or animals.