I think it depends on your genetics and perhaps a few other predispositions. The same reason 1 out of every 100 children is autistic. Unlucky genetics. Some kids end up with ADHD, Aspergers from the same toxic insult that caused others autism. While others are effected in more subtle ways.

My CFS started with a stress trigger (6 months of very extreme stress) bound with very high anxiety. Somehow the stress either releases mercury stores or fouls up the detox pathways enough to increase the mercury concentration over some threshold.