What caused many of the Moai at Easter Island to be destroyed?

Many of them were knocked down or partially destroyed when Easter Island natives began to fight between each other.

Explanation:

Natives at Easter Island were divided into clans that compete with each other by the island's natural resources. Their wars were so violent and destructive that they lead to their annihilation, the end of the forests and, eventually, the Moai statues. Even if there are still a good number of them, the great majority were destroyed or suffered some physical damage.

some of the moai were destroyed in the process of bringing them to their destination, but many others were toppled by rival clans.