IMO, I don't think they loaded that beam properly.
Surely, it should've been laid flat. This would've allowed the beam to have a couple of chains holding it to the tractor end and the dolly end in order to prevent latereal movement.
Because there was such a tiny surface area resting on the dolly and the tractor, it allowed for too much lateral movement.
You can see that for quite a while before the incident, the dolly's wheels simply don't want to come back into line with the tractor unit.