Slash Pine with Witches Broom

A Witch's broom is a disease or deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest.

One example of this would be cytokinin, a phytohormone, interfering with an auxin-regulated bud. Usually auxin would keep the secondary, tertiary, and so on apexes from growing too much, but cytokinin releases them from this control, causing these apexes to grow into witch's brooms. Wikipedia