When traffic is backed up on a highway due to an accident ahead, people get frustrated and panicky about being late. It's understandable that they want to find a way around the lineup. When an ambulance or other emergency vehicle comes up the left side, people get the idea that the best place to be is right behind it. These motorists were all thinking the same thing. As traffic pulls to the right to get out of the way of the emergency vehicle, it creates an empty spot right behind.
The motorist with the camera has pulled into the left lane and has found it to be smooth sailing right behind the ambulance. Somebody has squeezed in between and is doing the exact same thing. He needs to keep right on the tail of the ambulance to avoid being blocked by every other person who is just as eager to chase it.
An Audi driver in the middle lane is getting ready to chase the ambulance too and he doesn't seem to care that he is cutting off the vehicle that was chasing it first. He pulls out and the camera car driver lays on the horn and the brakes to avoid hitting him. It's back to chasing the ambulance for all of them as the lineup of cars continues up the highway behind it.
In Ontario, as in most places, the person changing lanes has the full responsibility to do so safely, and in a manner that does not require evasive action by any other driver. The person already in the lane has the right of way. In this case, no accident occurred and it was not an issue. Following behind an ambulance can be hazardous because it is predictable that people will try to force their way in so that they can follow it through traffic.