When people hear that a dog bites a man, it's not really surprising because sometimes dogs can be angry or scared and they might bite. But when people hear that a man bites a dog, that's very strange and unexpected and kind of funny too. It's like if you saw a baby playing with a big toy that was meant for grownups, it would be funny because it's out of the ordinary.
To explain it simply, we know that usually dogs bite humans, but it's not normal for a human to bite a dog. So, when we hear that happening, we get surprised and amused. It goes against our normal expectations, just like if a fish caught a man instead of a man catching a fish. It's like a funny and strange news headline.