Why do my chickens bury the eggs in the straw?

[boyswillbeboys]

I don't know if this has been asked before, but I keep finding lately that my chickens (1year old) are burying the eggs in the straw of the nest box, usually it's just one box a day (of 4-5 different ones they like) and all the eggs in that box are spread out with a layer of straw on top.

does anyone know why they do it?