Loose bathroom wall tiles
I can see the ridges left by the installers notched trowel. Look at the back of any loose tiles. It should have thinset on at least 80% of it's back. I'm guessing the thinset wasn't mixed correctly. If it is to dry the tile does not get pressed into the notched adhesive so as to form a mechanical bond. You can check if the tiles are affixed properly by giving them a slight rap with your knuckle. You should hear a dull sound. A hollow or "ting" reverberation indicates a void or low spot were the thinset isn't fully supporting the tile.