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Does Sweet's undergo some kind of change after it has been applied that causes it to beome corrosive?
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The short answer is yes, it does...well, sort of.
You can make a very close substitute for Sweet's by mixing 2/3 janitorial strength ammonia (10%) and 1/3 thick, liquid detergent.
Ammonia is a base. Normally (some exceptions...ammonia attacks copper) a base will protect metal, as opposed to an acid which will eat metal. Some of the lubricants used on large rotating machines are so full of strong base that they will peel your skin and flesh off.
The problem with ammonia is that the ammonia gas evaporates out, leaving plain water. Water, of course, promotes rust.
The other problem is that copper in a barrel acts as "reverse galvanizing". Where zinc will protect steel, copper does the opposite, and promotes pitting and corrosion. Leaving water in the barrel forms a battery, with copper and steel, and promotes the pitting.