Vegan cakes can turn out gummy in texture when there's an excess of certain ingredients like flax eggs or starches such as cornstarch or tapioca starch, which can create a sticky, gluey texture. Additionally, overmixing the batter can overdevelop gluten in wheat-based flour, leading to a gummier consistency. Properly balancing ingredients, using suitable egg and dairy substitutes, and avoiding overmixing can help prevent the gummy texture in vegan cakes, resulting in a more desirable crumb.