Trees can die from the top down or the bottom up or from the middle either way, depends on the causal agent.
Dwarf mistletoe, which tends to affect conifers, and is species specific. That is, the species of dwarf mistletoe that affects Western hemlock is different than the species of dwarf mistletoe that affects Lodgepole pine etc.
It is a parasitic plant, that grows into the branches or stem of its host. It will cause swelling of the host part and will affect wood quality. In conifers, witches brooms are a common sign. These growths need to be considered when doing a tree assessment, as they tend to be significantly heavier and the branch wood is much more brittle.
Dwarf mistletoes spread their seed by building up water pressure within the seed 'pod' and when it reaches a certain point "spitting" the seed out. A good analogy would be spitting a watermelon seed. The seed is covered in a sticky coating and as it shoots out, it will stick to any foliage it lands on. If it lands on a proper host species, then the new plant will begin to grow. Seed movement can also occur by animals such as birds. Infection rates within a stand tend to move slowly as the seeds only travel about 50'.