Off the top of my head, he sold the game rights for a lump sum. Basically, he didn’t believe the games would be popular, so he didn’t believe royalties would ever pay out. So he demanded a large up-front payment instead of the standard “small payment plus royalties” deal that authors would go for.
Then when the games ended up being super popular, he came back and tried to demand post-dated royalties. When the company (obviously) refused, he got crotchety and started shit-talking the games to anyone who would listen.
Honestly, the games have been a huge benefit to him. Tons of fans ended up buying the books because of the games. But he’s salty that he demanded a bad deal, because he didn’t want to risk losing if the games flopped.