The question largely depends on the age of the children in question but I can comment on my experience with the under 4-year old crowd.

I agree, most children misbehave to test boundaries. But I think there is also a deeper emotional inexperience thing going on here.

For example, when a child hears something they don't like , there is a rush of emotion that is associated with that statement. As an adult know how to 'listen' to our emotions, weigh responses and respond appropriately.

Young children, often inexperienced and unable to label that which they are feeling, can just lash out or get angry or start to cry; all examples of what we might perceive to be 'misbehavior'.