I think he's not only smart but also invested a lot of time and effort into learning everything that comes along his way. If you have read his blog spots where he wrote about his methodology to hunt for certain bugs, he seems to have a good grasp at the primitive blocks that could lead to a bug. With that kind of understanding, he is more likely to observe weird behaviours and dig deeper into them.
Furthermore success breeds success. Once one hits a certain reputation, he/she will be invited to private bounty programme and live events, that's where low hanging bugs are more likely to be found. Collaborating with other top hunters also help one improve the hunting methodology to discover other bugs that one doesn't usually hunt for.
If i were to do bug bounty, I will either check out all the latest research or do some research for myself then hunt specifically for those stuff across different bounty programmes. Otherwise I would go for subtle business logic bugs or single-sign on authentication issues.