When the westerners talked to the natives they learned that the natives were trying to lure back the planes; because the planes held mana from the Gods, called "Cargo". Cargo was all sorts of magical things that the islanders didn't have or understand, but they wanted. They didn't know who the men were that tended to the planes or who were the priests of this Cargo, but they knew that if they mimicked them, that maybe they could lure the planes and Cargo back. The Cargo-cult, as they were named, built a religion around things that they didn't understand and on the fables of the people that had been there but couldn't accurately describe what they had seen.
I've always felt the Cargo Cult, and other religions were the exact same thing; Mans need to describe things that are beyond his comprehension.