This is pretty comparable to being asked to be critical of yourself (e.g. "What's your biggest weakness"). Generally speaking this is a bit of a trap and your main goal should be to not take the bait and move on to another topic ASAP. If they really press the issue focus on issues where there's nobody to blame and there's not that much that's going to be held against anybody (e.g. Old Co become very big which was fun but now I'm looking to be part of a smaller team). The key thing is to keep things positive and move on to why you want to work for them instead of why you don't want to work for your old employer.