Hes upset that other, living, victims of the collision got a larger share of the payout than he did.
It appears to be quite the other way around. The living victims were upset that he got a larger share of the settlement, and took him to court to get some of it.
I had to do some digging but the article is a little fuzzy on a few details. The at-fault driver only had the bare minimum coverage and his insurance company only contributed $100k. The other victim, Duncan, also had uninsured-underinsured coverage and it was her policy that kicked in the additional $800k. Herrin also had at least one uninsured-underinsured policy which covered his son.
He collected $1.6 mil from his insurance company (perfectly fine since it was his and he paid for it) and a majority of the $100k from the at-fault guy's insurance (again, not a problem since he suffered the greatest loss). The problem was that he was also given a huge portion (over $677k of the $900k total pot) that included a huge amount the other victim's insurance policy.
It is my understanding that the whole purpose of uninsured-underinsured policies is to cover those who aren't insured. Since the at-fault driver's insurance didn't cover expenses for all the victims, it fell to each individual's insurance policies to make up the difference. It seems odd that he would be able to claim a portion of the other victim's policy in the first place.