Could be that the first 'death' was faked and the second one was real. Or maybe it's a symbolic death and a literal one. It's all up to the author's imagination!
It could be a case of mistaken identity or a parallel universe situation. Or perhaps it's all part of a complex dream or hallucination.
Maybe it's due to some freak accidents involving flammable substances around him.
She might have suffocated if the sandbox collapsed on her. Maybe there was some sort of accident like a heavy object falling into the sandbox while she was in it.
Another possibility is a curse from the supernatural world. Maybe he entered a haunted place or broke an ancient taboo, and as a result, a curse was placed on him to transform into a dog as a form of penance or to teach him a lesson about respecting the unknown.
It might be that the man made a deal with a witch and broke the terms, causing the transformation. Or it could be a symbol of his inner transformation and represented metaphorically as a physical change into a dog. There could also be a virus or a strange potion involved.
It could be a mystery or a crime plot. Maybe he was kidnapped or went into hiding to escape something.
Maybe he got lost in some remote area. Or he could have been kidnapped and held captive by bad guys.
One possible reason could be a broken heart. If she was deeply in love and got rejected or betrayed, the emotional pain might be too much to bear. For example, in some stories, the girl loves a guy who turns out to love someone else, and she becomes so devastated that she loses the will to live.
Maybe he was trying to escape some disease or a dangerous situation. Or, it could be part of a scientific experiment.
One reason could be dissatisfaction in her current marriage. Maybe she feels unloved or ignored by her husband. For instance, if her husband is always busy at work and doesn't have time for her, she might be tempted to seek attention elsewhere.
Perhaps he is making a statement about gender equality. By wearing a skirt and heels, he is challenging the traditional ideas of what men and women should wear. It could also be that he simply likes the aesthetic of the skirt and heels. He may have seen them on someone else and been inspired to try them out for himself. Maybe he is part of a subculture that embraces gender - nonconforming fashion, and this is his way of fitting in and expressing his identity within that group.