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The Hell-Volhard-Zelinsky halogenation reaction halogenates carboxylic acids at the α carbon. PBr3 replaces the carboxylic OH with a bromide, resulting in a carboxylic acid bromide. — “Hell-Volhard-Zelinsky halogenation - Wikipedia, the free”, en.wikipedia.org
The reagent soup forms electrophilic bromine, which halogenates the amide nitrogen (so Cacycle was actually exactly right:). You're under basic conditions, so deprotonate the N to get an anion, which rearranges to form an isocyanate: carbon attached. — “Wikipedia:Reference desk/Archives/Science/2007 March 28”, en.wikipedia.org