What are probable causes of a person developing als?

ALS: Amyotrophic lateral sclerosis or ALS (lou gehrig's disease) is a central nervous system disease that causes progressive loss of strength and coordination. There are no known risk factors which predispose someone to developing als. Most commonly, the cause is unknown but in about 10% of cases, there is a genetic defect which leads to the problem.

No clear answer: Other than 10% familial incidence and the rare occurrence of a genetic variation, we have no good idea of causation in the 90% sporadic variety.