Engine acceleration when braking

I just purchased a beautiful 2013 silver x3 35i m sport last weekend. It's gorgeous. It now has about 140 miles on it. Twice when breaking slowly (in a parting lot) to stop, the car/engine has continued to try to accelerate. To the point I was basically standing on the break and it was still creeping forward. The first time, to avoid crashing into the building in front to me, I punched the start/stop button to turn the car off. The second time, the car moved forward to where I had to pull into traffic quite dangerously. The floor mat was not obstructing the gas pedal etc. so that's not an issue. The dealer now has it back to investigate. Any thoughts? Never experienced anything like this before. Thanks.