Too many people want to play traffic cop from behind the wheel while driving. Impeding the flow of traffic is also a violation. Slowing down starts a chain reaction that takes several seconds for following traffic to respond to.
The vehicle that slowed down to allow merging traffic started the traffic crisis.
It is the responsibility of merging traffic to do safely.
Sure the driver that rear ended the vehicle in front of him/her was driving too close to avoid the accident BUT the one that impeded the traffic that started the crisis was the one playing traffic cop. They are neither licensed or certified to do so. If drivers want to play traffic cop let them be trained and licensed to direct moving traffic, and to do so safely OUT IN THE OPEN WHERE EVERY DRIVER CAN SEE THEIR INSTRUCTIONS TO THE TRAVELING TRAFFIC.
What happened in the case of the OP was a private signal between two drivers, unknown to others, that involved many other drivers and their families who were left out of the communication. All their lives were endangered by the decision of the one who impeded traffic for another's rush to get on the highway.
Safety overrule courtesy on the highways. Playing traffic cop can and often does cause property damage, serious bodily injury and even death BUT the guilty one often times goes free.
Hopefully they have a conscience that will prevent them from continuing their bad judgment of trying to direct traffic from behind their steering wheel; on our highways.