I think it happened like this:

1) Arm of the backhoe was up high enough to hit the bridge, but not high enough to hit the retaining guardrail, etc. (1st picture.)

2) The rebar, etc., caused the backhoe and truck to suddenly slow down, lifting the backhoe off the truck bed (and breaking the chains, etc., holding it down). Bucket of hoe hits underside of roadway.

3) A piece of the arm (maybe a hydraulic reservoir - it's hard do tell what it is) wedged itself against the retaining wall and that's what caused the arm to raise and the forward progress to stop (coupled with the energy loss in cutting through the rebar and concrete). (Blow up the 3rd picture.)

Hydraulics don't usually act on their own, so how the arm ended up being too high in the first place is still a mystery. I'd have to suspect operator error.

FWIW.

Cheers,
Scott.