don't bad mouth the previous employer, say something else like:

even the former emploer is the good compnay, and you have good team there/boss is ok, and the projcets are good too, but maybe the project is not chanllege enought for you; or maybe that's not what you want to do (indicate the job you apply now it's exact you want), or maybe you do not like their practice, or maybe i don't agree their diretion, etc. could be any reasons that make you want move on......