As my rather speculative comment* has more votes than any of the answers, I felt I should try to track down some sources.
So first we take fabricate (Cambridge: especially US to produce a product, especially in an industrial process). That's clear enough in its relation to production. The example at that link is fabricating steel, which is made from raw materials.
A carcase (Oxford: 1.1 The trunk of an animal such as a cow, sheep, or pig, for cutting up as meat.) is easy to think of as a raw material in meat production.
The step I was unsure about is that of fabricating cuts of meat from a carcase. The (UN) Food and Agriculture Organisation, in their Guidelines for slaughtering, meat cutting and further processing (1991), say:
Nowadays more processors are removing the bones thus fabricating a boneless rump (chump) and a boneless ham.
This is a pretty clear use presumably by experts in the field of the sense of fabricating the end product from the carcase. Older results to illustrate this step can be found by searching google books for meat fabrication.
Jargon does funny things to the meanings of words, and the meat industry has a long history of moving towards less bloody terms (e.g. abattoir vs. slaughterhouse). Fabricate as a replacement for butcher (the sensational meaning V kill (a person or people) indiscriminately or brutally is quite common) fits this pattern. An instructive intermediate quote is
some UK meat cutters purchase head-on beef carcasses, open the skull to extract and merchandize the brain – for breakfast food – and then fabricate the carcass into steaks
Removal and handling of BSE specified risk material
G.C. Smith, in Improving the Safety of Fresh Meat, 2005 quoted here, emphasis mine
This is useful because it has the sense of fabricating the raw material into products; it's a small step from fabricating cuts of meat and a small step to fabricating a carcase.
A Google books search for "carcass fabrication", "fabricate the carcase", etc. find nothing, though there are plenty of web hits, suggesting that this use is fairly new. That said, Encyclopedia of Animal Science, Wilson G. Pond (Ed), 2004, does have a section entitled "Carcass Fabrication".
It's also worth noting that applying fabricate to the raw materials rather than the finished poduct isn't new:
As desiener of the Lone Star Steel Company plant in Dainger field, Texas, he is at present interested in trying to obtain for that company the approval of Pres ident Roosevelt for the expenditure of $40,000,000 with which to provide a fully integrated plant so that it can fabricate the iron ore it processes at its present huge establishment.
The Evening News from Harrisburg, Pennsylvania December 22, 1943, OCR errors retained
* I originally said:
It's unusual and new to me so I won't answer unless I can back this up with sources. Using fabricate to mean manufacture (from raw materials) is widespread. Then treating a carcass as raw material makes sense, so fabricating joints (etc.) means the same as butchering the carcass. The meat industry has a history of using less bloody terms where possible (abattoir vs slaughterhouse, stun vs. blow the brains out) so fabricate as a euphemism for butcher is a small step. Then all we have to do is apply the verb to the carcass rather than the joints, a move that's common in jargon.
