There is no radar sensor, it is purely done by the forward facing camera and as you had steered to change lane, for a moment you would have been heading toward the vehicle is the adjacent lane. Do you know how your distance setting is set? If it set at the longest distance it would be even more likely to have slowed your vehicle to keep that gap. To be honest on the M25 that I use on a regular basis, the 1 bar distance is too long and you get constantly slowed with vehicles moving into an inviting gap. Even thou most would say, to be safe you are actually already too close.
But obviously any vehicle traveling behind you would be at fault for not having left enough distance behind you to be able to stop, something may have fallen from that vehicle causing you to brake, the slowing would have been indicated by your brake lights.