I don't know, either. If it turns out that 2 or 3 out of every 100,000 people in the general population get ALS and that the rate for football players is 10x as high, then you've probably got something. But I don't know the numbers. You'd also have to account for other distinguishing characteristics of football players. Do they use PEDs at a higher rate than the general population? Again, I don't know the numbers.
All that said, I suppose it does seem intuitive that a disease that disrupts brain activity could be related to banging your head repeatedly, but that doesn't make it so. When I was a kid, everyone was convinced that repeatedly "cracking your knuckles" led to osteoarthritis later in life, but current medical wisdom is that this isn't so.
Donald Trump is a douchebag.*
* In order to avoid confusion and remain compliant with the board's Fair & Balanced requirements, the above statement shall apply to any/all posts critical of any political or otherwise public figures.