I don't know what it is about this question that drives some writers bonkers, but it's one we all get at book talks and literary events.
In response, some authors take the humorous approach ("In Aisle Twelve"), and others try to ignore the question outright ("Next!")
Some will roll their eyes and joke about banning the question before offering their response. Personally, I never really understood the reluctance because I love answering this question. I find it so much fun to tell people about the murder ballads that inspired my first novel, A Murder at Rosamund's Gate.
Indeed, I get a lot of my ideas--and questions-- simply by poking around the Early English Books Online. For example, I've been thinking a lot about poisons lately--because, you know, writer--and I came across this gem. The story from 1677 pretty much writes itself!
"Horrid News from St. Martins: Or, Unheard of Murder and Poyson. Being a true relation how a girl not full Sixteen years of age murdered her own mother at one time, and a servant-maid at another time with ratsbane. As also, how she very lately gave poyson to two gentlewomen that since met her Mother's Death kept and maintained her. Upon which being apprehended, she has confessed the former villanies, and was on Tuesday last the 19th of this instant June, committed to Prison, where she now remain."
This story may not make it into one of my novels, or who knows? It could be the crux of the whole tale. But the point is, ideas don't come fully formed, they come in bits and pieces. The important thing is what we do with those ideas--the questions we ask when we ponder different ideas. It's not just whodunnit--its all the other questions that drive the story forward.
I think, ultimately, that the idea is not a story until the writer learns to get beyond the "What happened?" and begins to ask "Why?"
But what do you think?