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

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).
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.