My guess would be that the Mexicas thought cyclical. Think about the Aztec calendar and the stories of their deities. I imagine this had to do with that connection. They mentioned the deformed heads, so the people on it very likely weren't just some random low class people.
I am quite intrigued if it was to like a "don't destroy us" kind of ritual to please Mitlantecutli, Huitzilopochtli or good ol' Quetzalcoalt.