They are random events that are perpetuated by the government.
http://minneapolisfed.org/pubs/region/99-03/cycles.cfm
I do agree that bush got the short end of the stick when he came into office, as after such a booming economy. However, some of the ways he handled it I disagree with.
Question for people in economics. Why do we cut taxes to the rich when we want to help the economy when their marginal propensity to save is higher than that of the low income?