This is a complicated topic, but we've written our detailed thoughts about it here: https://sundaysfordogs.com/fda-warns-grain-free-dog-foods-cause-heart-failure-dcm/

The short version is that, while the research is still unsettled, the leading hypothesis is that the inclusion of legumes and potatoes in high levels prevent the proper absorption of taurine, leading to weakening of the heart muscle (so we do not use any in any of our recipes).