So you have had four days of warm weather since that cold snap. The first, second, and fourth days they were normal but on the third day they stayed on the roosts. That doesn't sound like hypothermia to me. If a strong cold wind is blowing mine will stay out of it, maybe in the coop but not on the roost. If one or more chickens are bullying mine, some might stay on the roost to avoid them but that is not a one day thing, that would be over several days. The only thing that makes any sense to me is a predator scare.
I've never had a hawk attack send mine to the roost, though they will go inside the coop. The only time mine have gone to the roost was after a dog attack, and they did not stay there all day even then. That does sound strange.