Have you ever seen a thick bundle of twigs in a shrub or tree? These are "witches' brooms" -- the term apparently comes from the German word Hexenbesen, which means to bewitch (hex) a bundle of twigs (besom). The idea of a witch riding a bundle of twigs has waned, but the name remains, and maybe the idea too thanks to Halloween and Harry Potter.
Anyway, back to the bundle of twigs. Most any woody plant can form a witches broom, sometimes they are inconspicuous and sometimes obvious. I notice them a lot on wild highbush blueberry bushes.
Abnormal shoot growth results in this mass of twigs from a single point on the plant. The causes vary and include fungi, parasitic plants, insects, environmental stresses, and sometimes mutations. The broom on highbush blueberry is likely caused by a rust fungus, Pucciniastrum goeppertianum. Rust fungus have a secondary host, and in this case it is balsam fir. So, if you are trying to grow these two plants, best not to have them near each other.
Yet another fungus -- commonly known as yellow witches' broom -- infects balsam fir, with chickweed as the secondary host. Some witches' brooms are caused by mistletoes or dwarf mistletoes, parasitic plants that cause a similar reaction. You might see these in softwoods.
These are rather mysterious phenomenon, the causes are not always known for a given broom. A broom here or there on a plant will not kill it, so just another interesting part of nature's diversity.