What causes cancer pain usually?

Nerve involvement: Tumor tissues invade surrounding normal tissues, including nerves and that can cause pain. Tumor infiltrating the bone causes weakness and overt or micro fractures that are painful. Inflammation often accompanies cancerous tissues and may contribute to the pain.

Many reasons : Cancer pain may be caused by direct tumor invasion of nerves, pressure on neighboring structures, bowel obstruction, release of toxic byproducts, etc.