Hi there, Lee! Because this a recipe we aren't familiar with, it can be tricky to say! Xanthan gum is a binder that helps to give gluten-free baked goods structure and prevents them from crumbling. You can try adding a small amount (try 1/4 to 1/2 teaspoon) to see if that improves the results. Also, most often when a baked good collapses while cooling it's often because of underbaking. The strawberries likely added extra moisture, which will require the baking time to be extended. We hope this helps for future baking adventures! 

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