Baked items that rise and sink after they cool, are often over-mixed. Double-check the ingredients, and did you use fresh (not expired) baking soda? It sounds like maybe an ingredient was missed, or just everything was overmixed. As far as the crumb topping, was the butter cold when starting, and did it resemble coarse crumbs before topping it? Those are the first few things that come to mind. Double-check your oven temperature with an additional thermometer you place in the oven to be sure it’s running at the correct temperature.

Mine also turned out rather flat and sunk in after cooling. The crumble topping also just melted and crystallized around the edges if the muffins. The texture was not bready but more bread pudding. Not sure what I did wrong.

I’m glad they tasted good, the leavening in them is the egg and since it’s only 1 egg, I’m not sure why they turned out flat unless the oven wasn’t hot enough when they went it, they start rising right away.So be sure the egg is mixed properly.

All crumb topping recipes are not the same, this one stays nice on top of the muffins and doesn’t need “special” attention, that’s why I love it so much. And I made many versions to get it to stay nicely on top and not sink into the batter, or fall apart, or go flat or melt away when baked.