Our Syrian does this too, I watched him really early one morning back filling the hole to his home/bed and then he’s gone for the day, resting. At around 8pm when we open the cage door and start placing his food in, he’ll start moving it out of the way to come out and say hi or at times just pokes his head out then goes back in. I don’t move it out the way as I too received a little nip the day I thought I’d try to move it and see where he was curled up! Oops first time owner mistake
. I think it’s kinda cute they do this. I guess in the wild it would stop other animals getting into their burrows and the movement of the substrate also alerts them that something may be trying to get in. Your hamsters just making himself cosy and settling in.