We cannot say for sure what is going on with the holly.  Some reasons for leaf drop may be poor drainage in the area and/or drought which can affect the root system,  possible scale insects, etc.
It would be helpful to send digital photos of the holly, around the base of the tree,  and branches so we can see what you may be dealing with. 
mh

Here's a few pics. Hope they help.

Here's a few pics. Hope they help.

We don't see any signs of disease or pests in your photos. The good news is that most of the branches look alive, budded and ready to re-leaf the tree in the spring.

We can't see your exact situation here, but if there is a downspout headed right into that root area, it should be redirected. These trees don't appreciate 'wet feet' and decline or disease can become a problem. Alternatively, if it was very dry going into the winter there and the area gets lots of sun and wind which further dries out foliage, leaf drop could also occur.
We do think it looks as if your tree  may be planted too deeply, which can stress and kill a tree over time. There should be a natural 'flare' or widening of the trunk where it goes into the ground. We'd suggest gently pulling back the leaves and soil at the base and see if you can't uncover this area.
You could also consider having a certified arborist come out and take a look at your situation and offer solutions.

cm

Thanks for your assessment. I will look at the planting depth compared to the healthy specimen on the other side of the driveway for starters and clear away some of the mulch. Then we'll see how things go in the spring. 

Sounds like a good plan.

cm