I’m with bama1971. Saturated bed pushed up over the waterproofing. Good news looks like you found it before it was completely rotten. If you can find the tile again they can just cut the bottom 14-16” out. Remove everything from the area it will get dusty. Doors don’t stop dust FYI. I had this done 3 years ago, mine was bigger and a fancy pattern $2800. I’m in Shelby county where everything is higher. Probably would have been half at my lake house in Talladega county.