This is a tricky question because "anything you say can and will be held against you!" Be careful and only answer with a positive tone. You can say that you didn't find the job challenging enough for your skills, or that the workload was sporadic and you spent, say 40%, of your time on 'idle' mode. Never speak ill of your last employer because it shows that you will do exactly the same once you leave this one! Protect the image of your previous employer, and your current one will respect you for it.