Fire belly toads croak for what reasons?

If you mean croak as died there could be many reasons. One, they could have eaten something poisonous. Two, it could have dehydrated or not eaten enough. Three, it could have come into contact with a chemical such as bleach. There are many more reasons.

If you mean croak as in the sound they make, fire-belly toads usually croak when looking for a mate, warning another frog of a predator, or warning the predator it is poisonous.