Very interesting this happened to your cats. My cat Tim was on grain-free kibble both times he became severely constipated. The first time was Blue Buffalo Wilderness in chicken and the second time was Taste of the Wild Rocky Mountain Feline (salmon and venison, but really, mostly chicken). After the second time, when we realized we needed to make significant changes (we added a water fountain and extra water sources after the first time) and I started researching, I read somewhere that high-protein dry food is extremely dehydrating. I don't remember where, and I wish I could, but that has stuck with me. Both times Tim had to get cleared out, the vets told us it was extremely dry and like concrete, and felt the problem was dehydration, despite the fact that Tim was constantly drinking water.
Just to be clear, canned grain free food doesn't cause constipation in my cats. Just the kibble.