Bandura attributes aggressive behaviors to
the impact of observing models
the impact of punishment
a biological predisposition
random acts of violence

Bandura attributes aggressive behaviors to : The impact of observing models

Since we're little, we tend to observe the other people around us to determine how we should react to an event.
If for example, a child leave in a family that always act aggressive everytime small things happens, he will eventually grow up thinking that it's a proper way to deal with every small things that happen