Iconoclasm is basically just a fancy word for censorship. Often times authorities try to censor what they feel is appropriate or inappropriate for art and overall control art in general. Because art is a form of expression, there will always be art that others feel is not appropriate for audiences and there will always be people trying to tear it down. Art can represent feelings and emotions as well, which is why others might not like what it is showing.
One example of Iconoclasm in recent history is when the people in Iraq tore down the stautue of Sadam Husein. Sadam had raised that statue and demanded respect for himself. By having a giant statue, he was just showing how highly he thought of himself and was displaying how much power he had. After he was hung, The people tore down the statue to demonstrate that they were no long under his control. He no longer had any power over them, so the people destroyed his statue.
Art can create very powerful emotions in people and people will react, whether it is a possitive reaction or a negative one. Iconoclasm puts restrictions on what can and cant be displayed, so it causes more trouble than not having restrictions would.