This vulnerability could be a disadvantage in the beginning but soon developing a bigger brain became an advantage,
We can see in nature that vultures and other praying birds hunt weak/sick animals. Probably they did it with humans too and as I recall there were similar report from wars that birds prayed wounded humans still living but unable to defend themselves.
The question could be how many of these weakened human (child) were already dead but imho a seriously weakened person was in that times practically dead anyway. Attack from any animal bird or not just would speed up the process. There were a lot of animals that prayed humans when an opportunity was given. Even today hypopotamus, crocodile, tiger etc. are taking humans.
Actually this news should not been such a big news except for urbans never entering the "jungle out there".