Usually just happens. Leukemia is actually a rare disease, but because it is so devastating, those few cases make us much more aware of it. You can not catch leukemia from someone who has it and unless you have significant exposure to radiation or chemotherapy, your chances are quite low of contracting it. There are a few families where cancer and leukemia have higher incidences and genetic counseling should be sought.
Hard to say... Many times we don't know what causes leukemia, 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. Some viruses like ebv or htlv have also been implicated in certain types of leukemia.