If you're asking what causes ALS, JoelC has answered that. From what I've read it seems that there may be multiple causes of ALS, including a small percentage of familial (genetic) cases.
Dad has ALS. He has asked me many times what I thought he might have done to have gotten this, and even though I am an M.D. (as are many in our family) I have to tell him I have no clue. He thinks it might have been some insecticide he used back when, I repeat I have no clue.
If you're asking if you might have done something to have caused this to happen, I urge you not to. Sometimes, things happen, and there is no evidence that there is anything you could have done to have caused this, or to have prevented it. The diagnosed of ALS can give you such a feeling of helplessness---I went thru it when Dad was first diagnosed 6 yrs ago and I wasn't even the one w/ the diagnosed, but you are in no way responsible for this.