It's fairly common if you're in a room that has upper berths but only 2 booked, Carnival will move you out into a room without upper berths so someone who will use those upper berths can have the room.
That's one common reason why this happens. But as Blerk said they could move you for any reason whatsoever, or no reason at all.
I notice that when I book guarantee rates, I am often assigned a cabin that has upper berths even though I only book 2 people.