Why Do Movements Die?

Of late I have been thinking about "movements" and why most never last long. And by movement I do not refer the bowel kind, but to the kind that is often a result of some random act of perceived injustice. In the present age, movements are often associated with terms like "Activist" and "Activism".  Most activists I know or forms of activism that I have encountered are as a response to some external stimuli. Could it be that this one fact, the presence of an external stimuli working as a trigger, is the very reasons movements often die? Because, humans being what they are, once one overcomes that external stimuli one looses the motivation to fight it.