Handlebar wobble?

i have just fitted a rear luggage rack and a top box to my yamaha sr125.i have noticed that when i get some speed up that there is a slight vibration on the steering.when i lighten my grip on the handlebars it starts to shake.is it something to do with the rack and box weight or is it something to do with the airodynamics being altered.it didn't do it before the box and rack were fitted