Cannibalism is not a widespread feeding behavior among argonauts, although its not totally excluded especially when food resources became very limited. For more information about this subject, please refer to the attached article.
It could be due to stressed situation. It is likely that argonaut lay eggs inside the sharks stomach (if it was swallowed alive), and due to lack of space it easily can finish in the mouth of animal. Could you provide more details about condition of specimans in the stomach, size, etc.