They’ll feel better after typing in all caps.

Can’t confirm the quantity that would release, but a front-loading cement truck would certainly dump at least a small amount of concrete when slamming on the brakes that hard. The slump of the concrete seems to be 7-8”(loose) and there is no mechanism that would completely prevent concrete from spilling forward that I am aware of.

That said, the concrete does appear to continue for a while after reasonably assuming gravity would stop the flow. Maybe he turned on the hydraulics since he was pissed?