Hi June, Alexander did write a reply, which you can view by scrolling down or viewing the previous page of comments. I agree that this is most likely a case of allowing the dough to rise too long or at too warm of a temperature prior to baking. If the loaf pan you're using is larger than the size recommended, you may be tempted to let it rise higher in the pan than the structure of the dough can sustain. Another factor to consider is dough temperature; ideally you're aiming for a dough temperature in the 75-78F range as the dough rises. If your dough is much warmer than that it may rise too quickly, and be more susceptible to the kind of collapsing you describe. This article on desired dough temperature can help you regulate your dough temperature a bit more carefully, which can give you more control over the rising process. 

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