Crumbly means the crumb's structure is either too dry or too weak to hold itself. Four suspects, in order of likelihood:
Overbaking - even five extra minutes dries the structure into sand. Bake to a just-clean skewer and verified temperature, not to the timer plus anxiety.
Too much leavening or flour - excess baking powder blows the crumb open so wide it can't hold together, and extra flour (scooped cups instead of weighed grams are the usual villain) makes it dry AND weak. Weigh everything, always.
Fat not properly incorporated - fat is what makes crumb tender-but-cohesive. Rushed creaming or curdled batter leaves the fat unevenly distributed and the sponge falls apart along the gaps.
And the innocent one: cutting warm. A warm sponge ALWAYS crumbles - the structure only knits fully on cooling. Cool completely, better yet wrap and chill before slicing, and use a serrated knife with a sawing motion, no pressing.
Weighed flour, honest bake time, full cooling, serrated knife. Usually two of those four end the crime scenes 🙂
