Why do blood clots sometimes occur after surgery?

What causes life-threatening blood clots? What does the surgery do to us to make us susceptible to clots? Or are we always at risk for them, even without surgery? I have read of 3 or 4 deaths lately from blood clots 2 or 3 days after surgery where the person just died in his/her sleep. Why? I know exercise is supposed to help you not get them, and I know a clot is simply blood that's pooled long enough to stick together... but why is there 'pooling' at all? Any nurses or doctors out there with some answers?

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