What I heard was an alert go off after the first (red) car slightly veered off to the right, allowing sensors to detect the second (black) SUV's abrupt stop.
Are you sure you're viewing it with the sound in sync? There's no veering when I watch until well after the audible alarm. But if you look through the red car's tinted windows you do see the break lights of the cars in front - presumably it's that breaking action which the radar unit is picking up.
My take on this is the red car wished to switch lanes beforehand and was distracted by that, thus not reacting quickly enough to the breaking. The right indicator goes on and it slowly starts to move across just before impact (but not at a speed / timing which would indicate an effort to avoid impact)
