There are many reasons why a vehicle might slow down on a freeway, which is why you have to be vigilant of what is going on in front of you. It might be something laying in the highway, an emergency vehicle, a pedestrian, or someone changing a tire of a vehicle that has pulled off in to the grass, etc. Coming up to an interchange, you should always be more aware of what is going on because of the possibility that someone will be merging and that someone will either slow down or speed up to let this happen. In my opinion, the vehicle that struck the other vehicle in the rear was not using due care when driving on the freeway and hit a vehicle that had slowed down to allow the MH to merge.