They looked fine but weren’t. We had a fall that was dry and warm late…right up until Christmas. This is when evergreens are trying to take up and store moisture to survive winter. Most people don’t think of watering extra that late but that’s just what they needed. So, they went into dormancy without enough moisture stored. As soon as they thawed out, the problem became apparent. Another possibility is that mice or voles stripped the bark off all the way around the base of the plant which stops the flow of moisture and nutrients up through the bark. You’ll be able to see this damage.
Here’s a link to a blog post about this from a couple of years ago which should help you try and resuscitate them.