Hi,
My chickens do this a lot, it usually hapens because one chicken prefers to lay in a different spot and while doing so they like to make sure all the other eggs are underneath them.
To make sure my theory was correct, i tested it. Here's how it went:
Firstly I waited for a hen (bluebelle) to go in to the nesting box, waited for it to sit down and get comfortable,
Then placed a shop bought egg next to it, with in seconds she moved it underneath her using her beak
I then placed it on the otherside of the box ( one foot away), she stood up and went to get it and rolled it back to her spot and then carried on sitting on the eggs waiting for hers to pop out.
Hope I've helped
( Kept yard chickens for three years and made this account recently )