Dear Jason,
Sometimes when you unplug anything that has active power, such as Xmas Tree lights, you create an arc. Electricity jumps!! But!! You didn't state if the spark came from the connection or from behind the outlet cover; In which case might mean a loose wire at the outlet or you are yanking the plug by the wire instead of the male end of the plug itself. If you have a metal box, yanking the plug out by the cord may move the receptacle enough to contact the metal box ( Ground) and cause a direct short. In which case would cause the breaker to kick off. Turn the power off to the outlet and remove the switch cover. Examine the box and plug to ascertain that the receptacle is not cracked, there are no loose or bare wires, the recptacle is not loose in it's mooring allowing it to contact the box, and ascertain that the wiring entering the box is not loose as this may over a period of time rub the insulation enough to expose the bare wiring. If in fact the wiring is loose, PLEASE call an Electrician to repair the problem.
Let me know how it worked out.
Good Luck, Len