Rowling chose the pseudonym and the backstory very carefully.
Robert Galbraith, the author of The Cuckoo’s Calling, has now been revealed to be nothing more than an alter ego for J. K. Rowling, but that doesn’t mean the persona doesn’t have a fascinating persona regardless. For example, in case you were wondering how Rowling came up with his unusual name… well, she’s not too sure herself.
Not exactly the picture of a "military man"
But the Cuckoo’s Calling author did reveal part of the reason behind the pseudonym on his… her website – the page still bears the Robert Galbraith name.
Robert was apparently inspired by Robert Kennedy – a childhood hero of Rowling’s. As for the last name, the author can’t recall much, other than the fact that Emma Galbraith was her dream name as a little girl. How or why she learned it is a mystery even to her. “I don’t even know how I knew that the surname existed, because I can’t remember ever meeting anyone with it. Be that as it may, the name had a fascination for me. I actually considered calling myself L A Galbraith for the Strike series, but for fairly obvious reasons decided that initials were a bad idea,” the author explains.
The hoax has been called "The best act of literary deception, since Stephen King was outed as Richard Bachman."
As for her alter ego’s military background, which now seems like an obvious coverup, to Rowling (and her publishers) it seemed like the most logical idea at the time.
As Rowling explains: “It was the easiest and most plausible reason for Robert to know how the Special Investigation Branch operates and investigates. Another reason for making him a military man working in the civilian security industry was to give him a solid excuse not to appear in public or provide a photograph.”
Rowling had hoped to keep it a secret a while longer.