Bottom pan view of a rum cake I made. Similar thing happens when I mix ground strawberries as infusion in cake. What is happening here? Have heard of cakes collapsing from top but this is happening at bottom of pan. Please advice. 

Here's a good visual guide...Those dips are called "cupping" and have a few different causes. http://www.progressivebaker.com/downloads/Cake.pdf

thank you costumeczar!