What happens when cancer spreads to the spine?

Depends : Pain, fractures, and other secondary symptoms including mobility issues related to spread of the cancer. The management is related to the type of cancer and it's best to talk to the personal physician about the options available in any given situation.

The spine bears: The weight, and the force of gravity. A cancer infested spine has a penchant to collapse, causing pain, sometimes fragments or globs of tumors pressure the spinal cord. Treatment intends to relieve pressure and pain, likely surgery and radiotherapy.

Spine Metastasis: Usually nothing happens, actually. If patients with breast cancer, as many as 80% will have mets at autopsy. So, it depends upon tumor type. Lytic tumors do weaken the bone and could cause fracture (myeloma, lung, renal cell, thyroid). While blastic tumors are not at higher risk of fracture. Worst case is aggressive tumors that cause spinal cord pressure and possible paralysis. Pain precedes these.