Circuits are tripped when there is a power overload on that circuit. What this means is either something is drawing more power than the outlet allows or there is a short in the circuit. If it continues to happen you should consult with an electrician.
Also when certain appliances get old. That have motors such as vaccume cleaners and blenders and plug in fans. The motor will over heat causing a fire hazard. This will cause the circuit to trip off as a safety feature.