All ground wires do indeed get connected together and in the panel box it gets connected where all of the other ground wires get connected. The only way it would cause a short there is if you connected it to the fuse lug. Grounds do not cause short circuits. Shorts are caused by a hot wire touching the ground.
Also, it would help us diagnose the problem if we knew more about the circuit. Is this extra wire going to the switch or another light. You make it sound like the lights are wired in parallel and not in series. Where do the wires for the other lights tie together? in a junction box, at the switch, at the breaker???
Post again and let us know.