What are the causes of hematological malignancies? what is the leading cause for hematological malignancies, such as leukemia or lymphoma?

Mostly unknown.: Only a few leuekemias have known causes. There are certain genetic conditions that predispose one to getting leukemia. There are a few exposures (benzene, radiation, chemotherpy agents) that cause leukemia. But mostly, the causes are unknown.

Hard to say...: Many times we don't know what causes leukemia or lymphoma, irrespective of type. The process is probably multifactorial and has a strong genetic component. In a small subset of cases exposure to various toxins like paint fumes, or to radiation, can also cause it, but this represents only a small percentage of patients. There is also an association with certain viruses (ebv, htlv).