Well, 8,600 gallons, does that make a full trailer?
Not full would mean, if he reduced speed too late, and had to brake harder, the gas would move in the trailer, going back, then forward and moving the truck some meters forward.
It's not very easy to drive fluids or hanging meat.
I don't know much about the situation in the US, but my father told me about some of his colleagues who don't have much experience driving trucks, some of them only made their bus license and some years back they got their truck license with it, then get onto a truck with hanging meat (you need special licenses for dangerous substances), and very soon experienced their first accident.
Maybe some details get published soon.