Stop accelerating to highway speeds with your rear hatch window rolled down.. Simple as that.. If you have any your front windows down (sunroof included), and your rear hatch window down, a low pressure zone is created inside of the cabin relative to the air outside the top, and sides of your 4R. The turbulent mass of air at the immediate rear of the vehicle that is created as you accelerate is also a low pressure zone, but not AS low as the inside of your cabin. Because of this, a certain quantity of cabin air is sucked OUT through the front windows and the sunroof, and simultaneously, outside air is sucked IN through the back hatch window, from the same region into which the tailpipe exhausts.
There isn't anything "wrong" or broken. You either leave your back hatch window up while driving on the highway, or re-route your tailpipe to exhaust on the side of the vehicle so that your exhaust gas gets caught up in the passenger side slip-stream and trails away, as opposed to staying "stagnant" in the low pressure turbulence behind your car.