Q: Car abruptly stops in traffic, but engine still on - 2016 Subaru WRX

While driving at night (around 11pm) at about 30 mph, I was in 3rd gear and driving in a straight pattern when my car came to a grinding and sudden halt. I had not applied the brakes and since I had been moving forward in 3rd gear, my foot was not on the clutch pedal. The engine was still running when the car abruptly stopped and the engine sounded fine. I was in the center lane and facing the proper direction. I was ¼ - ½ a block away from where it had stopped itself. I am not aware if the brake lights had come on, to warn other traffic. I pulled off the road, turned my car off completely, restarted it, and drove home. Less then a day later it happened again. This time I was in 2nd gear at about 12-13 mph and in traffic. It completed stopped, like before, without any change in speed and no pedal movements. Again, I was in the center lane and the engine was still on, sounding fine as if nothing had happened at all. I pulled off and restarted the car. What is your professional advice on what I can do to get this fixed?