I seriously doubt the easiest way to get the hat on is by digging a massive hole, burying the statue, using the ground around it as a ramp, maneuvering the head into position and then somehow either raising the statue in position or digging out the ground around the hole so that it no longer looks like a hole. I think it's more likely the statues were laid flat, their hats attached and then raised with ropes and blocks of wood and a lot of man power.
Who knows why they were buried, perhaps their religions and customs changed over time and they buried certain heads as a mark of denigration or respect.
My bet would be natural silting and settlement. Easter Island was deforested at some point in its history. New groundwater patterns and a few hundred years could easily sink a few stone statues up to their eyebrows.
Who knows why they were buried, perhaps their religions and customs changed over time and they buried certain heads as a mark of denigration or respect.