This state is caused by a complex interaction between tumor cells, their products and host cells.
Tumor cells can directly activate the blood-clotting cascade.
Tumor cells can also inhibit normal blood cells from stopping the formation of a clot. Chemotherapy on the other hand can affect the bone marrow's ability to make cells called platelets. These blood cells help stop bleeding by making the blood clot. In this case the bone marrow does not make enough platelets. This can cause a patient to bleed or bruise more easily than usual, even without an injury.