FM2015

i just got an Audi A4 S Line multitronic 2014 approximately a week ago,it is my first automatic. Was driving around 30 miles per hour ,changed the transmission to manual via one of the levers on the steering,moved like half a mile,pressed the brake and it went into an emergency braking with the parking brake light on!!!! I have been trying to imagine what i did wrong because the whole experience was very very scary..