Maybe it's due to some freak accidents involving flammable substances around him.
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.
Another possibility is that the Avengers are being manipulated by a villain. The bad guy could be using false information to make the Avengers believe that Spider - Man is a danger. So they go after him without really knowing the truth.
Maybe he was trying to escape some disease or a dangerous situation. Or, it could be part of a scientific experiment.
Maybe it's to explore the theme of survival and resilience. A person facing such extreme situations could show how strong the human spirit can be.
Maybe it was an accident with some flammable substances nearby. Or it could be a magical or supernatural event in the story.
Perhaps he lost a bet. If he had made a wager with his friends or family members and the condition of losing was to wear a saree at the wedding, he would have to do it to keep his word.
Magic or spells. In many fictional worlds, magic is a powerful force that can cause all kinds of transformations. If a magician casts the wrong spell or if there's some sort of magical accident, a man could turn into a dog.
One possible reason could be mental distress. He might be suffering from depression or some other psychological disorder that made him feel hopeless. Maybe he had financial problems, lost his job, or had relationship issues.
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.
Betrayal could also be a cause. In some fictional tales, if a man betrayed his comrades or a powerful entity, that entity might use magic to turn him into a statue. It's a way of imprisoning him in a form that can't interact with the world anymore. For example, if a knight betrayed his king and made a pact with the enemy, the king's wizard could turn him into a statue.