No one physically made the children attack the Bobo doll. If anything, this experiment shows that Sigmund Freud's theory of aggression rings true.
Freud's theory of Eros (life) and Thanatos (death) is that we are genetically wired with a set life instinct and a set death instinct. 
While the life instinct drives our want to make connections, our death instinct is what drives our destructive tendencies.
These destructive tendencies include aggression and negativity.
Freud would argue that the children in the aggressive groups had a predisposition to aggression, thereby learning it through passive observation. When the model was being aggressive toward the Bobo doll, the children were not made to actively be aggressive as well.
Therefore, their latent want to be aggressive was awakened by being shown it was acceptable to be aggressive toward the doll. This shows that Nurture does not have any hold where Nature was not already present.