They were all, including truck 1, queued up at the stop line waiting for clearance to cross. Truck 1 received clearance to cross, he began crossing, then received instructions to stop after it was too late.
The rest of the emergency vehicles were stopped because they hadn’t been authorized. Truck 1 started moving because he had received specific instructions to do exactly what he was doing.
I take it you’re not a pilot, controller or someone who has ever worked an aviation radio?
FWIW the whole group received permission to cross. The instructions were to "Truck 1 and company", not just Truck 1
"Truck 1 and company" were cleared to cross. A few seconds later, "truck 1" was instructed to stop.
Edit: Confirmed truck 1 was the one involved in the collision. Previous text: It is unclear which truck specifically was involved in the crash. In photos, the truck has the number 35 on it, not sure if that would preclude it from being identified as "truck 1" verbally.