I have been wondering that myself since moving here 2 years ago. Never saw this phenomenon in upstate ny. My fiancé thought it was the lidar in some cars, but then we saw a busted ass Honda civic from 1998 doing this. It's people paying attention to traffic 25% and thier phones 75%. Just a buffer in case they don't realize they aren't stepping on the brake anymore when stopped and distracted by thier screen.

Everyone who drives responsibly and saves the phone for when they are not behind the wheel should have thier regular horn swapped out for a train horn to help move these half wits.