Unfortunately, we lost this 'Nellie Stevens' Holly outside of the Clubhouse. Hollies seem to be very prone to winter kill in Highlands and for that reason, I tend to stay away from planting them.
As an undergrad at Penn State, I worked for my professor and mentor, George W. Hamilton, Ph.D. We studied winter kill on turfgrass species. Creeping Bentgrass is very cold hardy and can with stand temperatures of -50* or more. Poa annua on the other hand is very prone to winter kill as it is not as cold tolerant. Our goal was to study exactly what was happening in the plant and why. I built several growth chambers where I was able to cover turfgrass plugs in ice. What we learned was that in the case of turf, it was a single event that cause the death, not a prolonged exposure to cold. Take a Poa annua green that was abused all summer long and goes into winter in a weakened state. Perhaps a warm spell hits an area for a few days and carbohydrate production increases followed by a rapid drop in temperature into negative digits. This one event would be enough to kill the Poa annua. We could not duplicate that kind of death on Bentgrass.
Back to the tree now...Highlands is known for up and down temps through the winter months. It is 60* right now but it will be 20* in the morning. These kinds of temperatures can wreak havoc on plants. The microclimate they grow in also plays a roll. If it is a shady environment, they most likely headed into winter with low carbohydrate reserves. Other hollies growing in full sun were able to fend for themselves. While winter kill is not fully understood, every year we see one plant or another succumb to it!