It sounds as if there is a short or a ground fault somewhere on that circuit, possibly related to the two lights that are also on that circuit.
When a circuit trips it is because ungrounded conductor wire (typically black, red or white marked) somewhere on the circuit is making direct contact to the grounded conductor (white wire) or the equipment grounding conductor (green or bare wire) or metal appliance. This allows the electricity to take a shorter path back to the breaker box without completing the whole circuit. This can be a hazardous situation so the breaker trips to protect you by preventing shock or fire.
You can begin by removing the lights and the switch plates to these and inspecting for frayed, loose or disconnected wires on receptacles or connectors like wire nuts. Be sure to turn the breaker off and test with a proper voltage detector before attempting this to be safe. If the problem doesn't look immediately apparent then you may need to call an electrician.