The most appropriate answer to this question is we dont know yet.
Unlike most other cancers, where we have at least some knowledge of the triggers that lead to cancer formation, in Hodgkin Lymphoma we are far away from the truth.
Perhaps the closest weve got so far is to find an association with a viral infection. Some studies have found that infection with the Epstein Barr virus (EBV) can increase the risk of developing Hodgkin lymphoma. This doesnt mean that you will develop lymphoma if you have an EBV infection. It means that your chances of getting Hodgkin Lymphoma are higher than someone who doesnt have this infection.
A recent report from the UK has found a possible link between smoking and Hodgkin Lymphoma. Smoking may interact with EBV infection to increase the risk of lymphoma. On the other hand alcohol seems to have no impact on developing Hodgkin Lymphoma. Find out more about this report in the article Smoking and Hodgkin Lymphoma Risk.
The other known fact about Hodgkin Lymphoma is that it probably has something to do with your genes. It has been seen that siblings of those with Hodgkin lymphoma have a 2 to 5 times increased chance of developing the same disease. Children of patients are also at a higher risk, though the risk is not as high.
In spite of discovering a few associations, we still have no idea what factors actually trigger the conversion of a normal lymphocyte into a Hodgkin lymphoma cell. Scientists are still on the lookout for a breakthrough.