From my experience, which again I concede is limited, horses are exactly like children, they are as naughty as they are allowed to be.
That's why they will act up when an inexpereinced or frightened rider gets on board, they play up because they can.
I was on a day out with a mix of experienced and amateur riders once - my horse decided to try it on. As we walked past the open gate to a massive field, he stopped and had a good long look at it, and I am sure he would have bolted had I not pulled him round and gently kicked him to get him walking. From then on we were fine, we both knew who was in charge.
Not so the most timid rider, who was frequently almost pulled over her horse's head as he bent to eat grass every few minutes. He finally decided he was fed up with his frightened rider, and he rolled, almost crushing her leg as he did so, she was lucky enough to scramble clear. The stable owner yelled at him and punched him on the shoulder, which he competely ignored. She admited that he was simply playing up, and needed controlling. I swapped onto him and the timid (and traumatised!) rider was led by the owner as we headed back. The naughty horse was perfectly behaved for me - I believe they sense when a rider is not in control and play up, like children.