Many questions regarding wobble on SP125 after 70 kmph

I am getting wobble on the sp after 70 kmph, it wasn't very intense but enough to throw me off because I've never experienced it before. It has happened twice so far. My question is, could this be because my bike is defective or lightweight commuter bikes like sp inherently tend to do this at higher speeds? or is it because of poor riding technique (i am a relatively new rider), is there a certain posture that you should adhere at higher speeds? i was just sitting upright.