if you back end is stepping out thast is pointing more towards oversteer if anything noty understeer. an uprated rear bar will reduce understeer. possibly inducing oversteer on lift off of the throttle.
if your car isnt new, it could be that it already has an uprated rear bar, the rear tyres might not be as good as the front tyres, the rear geometry could be out. ideally you will have the same brand of tyres on the front and rear.
if you dont like the back stepping out, then in my opinion dont just do the rear bar , instead go for a front and rear ARB package upgrade from the likes of eibach
if the car is new, then id say its unlikely to be the above but id go for a free alignment check to see if they are with in spec otherwise it could be down to driving style. i can step the back end out on mine if i want by lifting off in fast corners.
id