Just like many similar incidents of forgetting things, I bet he used a checklist but something distracted him at a critical point and he missed a crucial item. Forgetting to hook in normally happen when someone unhook from some reason after this step was checked, and forget to hook in again. When I watched first time
I was sure he will attemp a top landing near the houses or in the trees. The part in the article where he was lifted by the wind when attempting to land early is obviously nonesense. He choose to continue to the bottom from whatever reason.

Ramy