It could be that he had a past experience related to a dog. For example, he might have been saved by a dog when he was a child or had a special encounter with a dog that made him feel a connection. This connection then drove him to save the stray dog from drowning.
He might gain a new friend. The dog could become a loyal companion to him. It could bring more joy and love into his life. For instance, the dog could be waiting for him at home every day after work, wagging its tail, which would make him feel really good.
One reason could be that the dog is feeling threatened. For example, if the man accidentally steps into the dog's territory, like its yard, the dog may bite to protect its space.
One possible reason could be self - defense. If the dog was attacking him and he had no other means to fight it off, he might have bitten the dog.
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.
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.
One reason could be unhappiness in the marriage. If she feels constantly unloved or unappreciated by her husband, she might be more likely to look elsewhere. For example, if her husband is always critical of her, she may seek affirmation from someone else.
One can immediately throw a floating object like a lifebuoy or a log towards the man. This gives him something to hold onto and stay afloat.
He might not be a good swimmer and overestimated his ability. Maybe he entered the deep part of the river without realizing the danger.
One possible reason could be that Snape still has an underlying sense of loyalty to Dumbledore. Since Dumbledore cares about Harry, Snape might step in to save him from Umbridge's cruelty. He doesn't want to see Hogwarts turned into a place of unjust persecution under Umbridge's regime, and Harry is a symbol of the resistance against that.