Why can bread keep expanding in the oven after the yeast stops working?

Answer: Heated gas expands and evaporating water adds vapor to dough bubbles

Heated gas expands and evaporating water adds vapor to dough bubbles ✓ — Gas already trapped in the dough expands as it warms, and water evaporating into the bubbles adds more gas. These effects can enlarge the loaf after yeast activity stops. Eventually the dough’s structure sets and limits further expansion.

Yeast keeps fermenting even after the cells die — Dead yeast cells cannot keep fermenting to produce new gas. Yeast can work faster early in baking, while the dough is still warm enough for it to live, but expansion can continue after heat stops that activity.

Heated flour swells enough to lift the loaf without gas — Flour does not lift bread simply by swelling without gas. Heating helps starch and proteins set the structure around expanding bubbles; gas expansion and water vapor provide the lift.