What exactly does the verb "fall" mean when it is used for a cake?

Question

What does "fell" mean in this sentence?

When my mother tried it [raisin pound cake], the cake fell almost every time.

Answer 1

When baking, the cake or pie may rise due to air bubbles expanding due to higher temperature (air expands), sometimes if one is not careful the cake may fall (become flat), this may be due to the structure of the cake becoming unstable. It may also happen if the yeast, which is what makes the air bubbles, is outdated.

It's when you get this

Instead of this

_{(source: bbcgoodfood.com)}

Most famous for falling are souffles which are very fragile and easily disturbed by changes in temperature or vibrations.