I am not sure about why the carburnize will do it. But austenitic structures can be made magnetic by cold work.
The example I saw was a round bar that was flattened between two rollers - the more the strain the more magnetic it became. However, heat should remove the magnetism as in the carburize - so I don't know what exactly is causing your situation.
Carburizing effect that jdf is referring to must be related to 310 parts used in a carburizing furnace. They are found to scale heavily,become brittle and also exhibit magnetic properties. An analysis of carbon in such sample has given a result of 4-5% C with no Ni depletion and Cr remaining fairly constant.
Most people believe that carburization of nickel rich alloys is due to the depletion of chromium from the alloy matrix. Chromium is depleted from the matrix because various chromium carbides form in a carburizing environment transforming the remaining metal into an iron-nickel alloy. Iron-nickel alloys are ferromagnetic.
Note that the bulk chromium content of the alloy may not change, but the composition of the alloy matrix will be significantly lower in chromium.
