Probably no rear legs on trailer or not used as only 8t weight machine so not usually worth the bother. The trailer has pivoted over the axle and lifted the 5th wheel and braked axles on the unit causing the runaway. I was warned about this years ago by an old hand driver showing me the ropes but we’d only brake the trailer when loading stuff heavy enough to lift the trailer - 20t machines.
Probably a freak set of circumstances that caused it to happen here, best cause of of action to either shove the machine in reverse and off the trailer quick or give it some beans and get it on properly to drop the braked axles back on the road. If not aware this can happen then natural instinct would be to get out and run after it.