Since nitrofurans are banned e.g. in the EU from use in food-producing animals, but not entirely as medications for humans, there have been some stories/reports of possible diversion of human-intended medications for animal use, e.g. in Portugal in the previous decade.
In 2018 the USDA ran an investigation apparently prompted by the import ban of a shipment of US chicken on such grounds, but concluded that the semicarbazide (SEM) byproducts were likely not from the animals being treated with nitrofurans but "by-products formed during food processing".
While the detection of SEM in pre-intervention samples could have been indicative of nitrofurazone use, its absence in pre-intervention samples in this study suggests that the subsequent detection of SEM in the sampled products is not indicative of nitrofurazone use and may be a result of by-products formed during food processing.
However the paper is merely discussing sampling and testing, but offers no more detailed mechanism of how semicarbazide could have resulted during food processing. So, how could that have happened?
(I see that azodicarbonamide (E927) is a flour treatment and this breaks down to SEM as well, but I haven't read of this being ever added to meat. E927 is banned in the EU too, but not in the US. Slightly more usefully, there's a paper that discusses SEM occurence in some processed dairy products (whey & milk protein concentrates) and crustacean products (shrimp and prawn powders). Although not mentioned in the abstract, inside the paper, chicken powder and egg powder are discussed/measured as well. The [chemical] gist of that seems to be "The reaction of urea with hydrazine, favoured at higher temperatures > 80°C, can lead to the formation of SEM." Still I'm not sure how/if that has any relevance to raw chicken meat, which is what the USDA tested.)