As many posters have said, the majority are a small set of groups just churning out new coins on a production line, persuading new people to mine them until they're listed on an exchange, then dumping and disappearing.

The next biggest group are people who go "You're clearly doing it wrong, get out of the way" and launch a coin, often therein discovering why something was done one way or another.

Another big group is those who launch coins to see how they work (this sounds like a good idea, but isn't, it's a huge amount of work and it's a lot easier to try working with an existing coin).