Market sentiment effects it but I don't think it is usually the trigger. Wikipedia doesn't have anything on triggers but I remember my econ textbooks taking about it and there is usually one thing that pushes it over the edge like oil shocks or the housing loan defaults.

Expectations not being met causing loan defaults causes economic contract. When expectations are not met, then loans begin to be defaulted on and revenue estimates are missed, and future expectations need to be tampered, hence contraction as they are pulled back into reality.