What caused this ???

Hey guys. this all started with a squeak coming out of my wife’s rear end ( insert joke here ) and decided to take a look. I removed rear lug plate cover and then removed rear cover and saw the wear marks on the drum cover. The drum cover shows the cracking and the overall texture is rough from the cracking as seen in pics
I plan on replacing the brake shoes and drum cover ( on order ),

Any input would be helpful, thanks

I tried to attach files hopefully they come thru

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