I think it was because they were so close to the trees that were turning into Roman Candles. The heat had to be incredible. In their case, they were at risk of perishing from the heat in standstill traffic. If that is the case, nobody can blame them.
You can see the ash falling onto the hood of the car with the dashcamera. He also pulled out of his lane because of the heat.
If you watch this video you will see what was going on behind that vehicle and why those cars (and the motorcyclist) did not worry about blocking the fire truck. They were in trouble.
https://www.youtube.com/watch?v=PCc1FvZ3g0Q
You can also watch the small fire in the shrubbery at the house on the corner erupt into something much bigger.