I was turning west (left-hand turn) from a turn lane in the median, across three southbound lanes. I initiated the turn from a stop, after waiting through a traffic light. No nearby oncoming traffic. One car behind me in the turn lane. Halfway through the turn, the car suddenly and aggressively applied the breaks. Nearly caused the car behind me to collide with my rear. No evident reason for the car to stop. This is the second time something like this has happened with the ‘pre-sense’ breaking system.