This looks like normal interior needle loss. The needles on evergreen trees have a finite lifespan, of several years. When their time is up, they turn yellow/brown. This typically happens in fall. Our winter winds should blow most of the dead needles off the tree and your tree will look better in the spring. Some years this is more obvious than others. I'm not sure why you are seeing this on just one side of the tree, but if the tips of the branches are remaining green then there is likely no cause for concern.