Quite often it's not that they forgot to lower the bed (otherwise, they would've been hitting other overhead bridges, signs, traffic lights, wires, etc before they get to this point). But rather, they left the "PTO" (Power Take Off) button on, and at some point either accidently hit the switch which raised the bed, or a malfunction raised the body.
But...it's easier just to say they forgot to lower the bed on TV (or radio, traffic reports, etc), because that's what people understand.