There are many ways you can get ALS, and we haven't figured out most of them, but we know that NFL players, for example, have a greater risk of developing ALS than the regular population. What appears to be happening in some of them, according to Dr. Ann McKee, is that they're having changes in their brain due to the CTE that are overly affecting their upper spinal motor neurons, which appears to cause these ALS symptoms and motor symptoms. It's maybe one of 20 ways you can get ALS, if not more.
On February 6th, 2019. See this statement in context.