Why did you leave your previous job?
(Or "why do you want to leave?", if they are currently employed.)
As an interviewer, I want to get at the story behind the resume. If I see someone who has hopped around from job to job, I want to figure out why. The most common explanation is that the candidate talks a good enough game to get hired, but is a poor employee - either not very competent or has other issues (hard to get along with, poor attitude, bad work habits). If they don't fit that profile, then I want to figure that out.
On the other end of a spectrum, if someone has been in a previous job for a long time that is usually a good sign, but not always - it could be that they were in a seriously dysfunctional organization and were good at playing politics, or worked for an employer that was very slow to clear out dead wood.
Unless you are such a superstar employer (via pay, prestige, or both) that you can lure good people away from your competition, your best shot at making really good developer hires is to snap up talented people who have been let go through no fault of their own (usually due to a business failure, takeover or big reorganization), or who are looking to make a big career transition. The tricky part is separating these few precious kernels of wheat from the oceans of chaff.