Revolutions tend to occur when the basic needs of people are not being met. This means access to food, clothing, shelter, etc. This creates the right environment for revolutionary leaders to incite revolt. Another element tends to be prolonged violation of basic human rights.Most revolutionary leaders tend to exploit a bad situation and appeal to the masses and what they are not able to access in order to gain sympathy for their cause. This does require some charisma and these leaders often become folk heroes.
Some theorists (such as Karl Marx) theorised that revolution was a natural progression and reaction against the exploitation of workers in a capitalist society. To this end, revolutions often start in the name of a particular cause but do not always follow through.
I like the previous assertions, and in particular post number 5. Throughout history, revolutions have resulted in some level of dissatisfaction. If we were to look at this philosophically, voices were either being relegated to the margins or being outwardly silenced, and change was demanded. There comes a point in experience when people feel that a point of no return has been reached, and a point where people say aloud, "Enough." The ability of people to overthrow their daily economic and political lives arises out of their ability to envision what can be as opposed to what is. They do this in accordance to their perception. This is what causes revolutions in all forms- intellectual, scientific, technological, political, moral.
I would only add that another element is the lack of hope, hope that their lot will improve, and hope in the fact that they will be able to gain enough "power" to gain some control over their economic/political lives, perhaps through elections. I am thinking of the situation of the poor in our large cities at the turn of the century. Perhaps we've never had a revolution in this country because people are so spread out that uniting for a revolution was difficult at the turn of the 20th Century. I really think it's due to the fact that upward mobility has always been a part of the American economic system. We have a large middle class, and entry into that class remains possible --- through hard work, some good luck, whatever. If that hope were taken away, if we had a large underclass that had neither money nor power, then I think we would see more revolutionary activity. There were communists and anarchists during the "Guilded Age," but it never really caught on. I believe this is why we are so aware of maintaining a functional middle class through our economic difficulties today.
On the other hand, how would you have a revolution in a country as large as ours? We haved mini-revolutions now and then (Republicans out, Democrats in, Democrats out, Republicans in .... ) and perhaps this is enough ... and maybe this was the reason that "hope" was a large part of the Obama campaign ....
Revolutions occur when people who are oppressed organize and decide to take a stand and stand up for themselves. They occur because ppeople are at some sort of disadvantage and do not like the status quo.
Revolutions to overthrow despotic rulers and governments takes place when the oppression becomes so high that large number of people find consider it better to take the risk of opposing the people in power in an organized way. Of course there has to be some means of creating awareness of belief among the people that it is possible and worth while to overthrow the oppressor. Also there needs to be some kind of leadership to channel the energies of the people to fight the oppressor.
When the conditions are ripe for a revolution. even a small event can act like a spark to start a big fire. If at such time effective leadership comes forward the revolution may continue effectively. Otherwise energies of people may be frittered away in directionless efforts.