If they weren't getting enough to eat, that could certainly have been causing the problems and the deaths.
It sounds like you what you just did will be a good solution (separate pen for the thin ones to eat in peace).
I assume you will be checking regularly in future, to make sure this doesn't...
:( I'm sorry to hear that.
For the ones still alive, you say they are energetic. Does their poop look normal? Do they seem to be eating and drinking normally?
Any injuries on any of the dead ones? A dead animal can get stiff and feel cold even if it died of something completely different. If a...
Sleeping under a hen should keep them warm enough, which makes the cold ones even more puzzling. I wonder if something disturbed them so they moved around during the night?
I'm guessing because your chickens had trouble with the cold, and huskies do well with cold.
Personally, I suspect what you really need is either a change to the coop, or a slightly different kind of chicken.
Where do you live?
How cold does it get?