I would agree with the not getting the tube in the lungs, the end would have been very quick, imagine drowning with a teaspoon of water in your lungs.  The tube was in the right place, just too late.

My "guess" is shock to the system -- the stress of an esophageal feeding to an already stressed calf, not to mention going from dehydration to sudden introduction of 1-1/2 qts of milk.  But at that stage of the game -- there was about a 99.999% chance nothing could have been done to bring the calf back to normal anyway.

From past experience, if the dehydration is very noticeable and it's a certainty the calf hasn't nursed, you're fighting a losing battle.  The window for colostrum is usually closed before dehydration is easily detected.