Coccidiosis can be transmitted by hands, shoes, using the same brush or sponge to clean out feeders and waterers. If I have chickens in a cage there will be other chickens climbing on top and sometimes droppings will get in. I am not saying that is what the cause is for sure, but it would be...
How sad, sorry for your loss. I would suspect either they have coccidiosis, or that they had too much of a draft on them in their cage. I don't have any trouble with cold winter with silkies and d'uccles, but I make sure they are in a draft-free area with 2 X 3 roosts to keep their feet warm...