I've heard it explained that "hey" used to be more of an urgent way to get someone's attention, rather than a casual "hello" like it is now, so it sounded rude to some older folks.
Exactly. I thought it was just a silly joke to open up conversation.
In Germany we have something similar. Our word for Hey, "Hai" actually has two meanings. Obviously it means "Hey" but also "Shark"
So it was common to respond with either "Where" or the more famous "Fish"
If you went for Fish it turned into a silly game of trying to compound the word as much as possible in responses to each other. Usually going like "Hey" "Fish" "Fin" "Soup". Sharkfish fin soup