This is usually caused by three things.  The first reason could be there are too many electrical items plugged into the circuit.  Circuits are designed to only handle so much electric demand.  The second reason is there could be a short in the wiring, or the third reason is the breaker or fuse could be faulty.  It is recommended that an electrician evaluate the problem to ensure there are no safety issues.  Your electrician can also talk with you about adding more circuits to your home so that your electrical system can support all your needs.