Only response I have to this is they are wild animals with instincts. Some of those instincts will never be understood.

Maybe the siblings were able to determine that there was physically something wrong with the victim? Maybe it was a mental thing where either the victim was trying to be a bully, or the opposite where it was a victim of the bully?

Could also have been that the victim died or was dying for whatever reason.... it is very common when an asf dies to get partially eaten by its cage mates.

I think only mother nature could answer this one...