sounds like tramlining to me,the old classic mini i used to have had 13" wheels and the Pirellis they wore always seemed to have a mind of their own because they were relatively wide with squarish shoulders and i get a similar sensation with the 17" tyres on my current
MO and the one i had before that, also with 17" tyres,particularly under braking going down hill i notice it and the steering wheel seems to want to snatch from your grasp,the problem occurs on rutted roads or white lines i find,i'm not giving a solution admittedly but if you are concerned i wouldn't hesitate in going to the dealers.Good luck