Common carriers stand in situation of insurers of the goods, not only against the disappearance or destruction, but against all forms of damage to the goods except few limitations mentioned above.
In Forward v Pittard (1785) 1 Term Rep 27 per Lord Mansfield at p.33:
Now what is the act of God. I consider it to mean something in opposition to the act of man: for every thing is the act of God that happens by His permission; every thing, by His knowledge. But to prevent litigation, collusion, and the necessity of going into circumstances impossible to be unravelled, the law presumes against the carrier, unless he shews it was done by the King’s enemies or by such act as could not happen by the intervention of man, as storms, lightning, and tempests.
Nugent v Smith (1875) 1 CPD 19 at p.34 per Brett J:
Loss or damage arises through act of God where it is caused directly and exclusively by such a direct, violent, sudden and irresistible act of nature as the carrier could not by any amount of ability foresee would happen, or, if he could foresee that it would happen, could not by any amount of care and skill resist, so as to prevent its effect.