Check the park brakes have been applied, also question if these are runners or are they non runners going for repair or scrap…if non runners its possible (even if the height of stupidity, and yes this was suggested recently by someone 
) that the brakes have been wound off to allow the units to be winched or pushed up onto the bed.
Hopefully its a proper truck or plant transporter with high load capacity anchorage points in the deck itself.
Obviously the most forces will be exerted under any hard braking you might encounter, but make sure there’s adequate restraints pulling forwards too…common sense applies here, yes i’ve seen 4 x ratchet straps applied all pulling a car the same (wrong) way before 
and this from a bloke without a minute to live who’d been doing the job (badly) for 20 years plus, and yes that resulted in lots of damage once he got moving when the car in question jammed up between the prime mover and cars on the trailer on a turn.