One report I heard said that someone (security?) went up to the roof to confront the shooter before he made his shots. The shooter pointed his rifle at this person who then backed down for cover. This might explain why the snipers got a fix on him so quickly.

One ex-USMC commentator on Bloomberg said that the shot wouldn’t have been very difficult at that range. As to why he missed, maybe the shooter had to rush his shots since he was spotted. Maybe he was just a bad shot. It was explained that he wasn’t accepted into his high school shooting team.

This is what I remember hearing while doom-browsing YouTube, so take with a grain of salt.