I suspect it's not a "habit" at all. It's common with people who have dental appliances- braces, dentures, even tongue piercings. It's usually because your body knows there's something foreign in there and increases saliva production in order to digest it. Most people become accustomed to it and their bodies stop producing the excess saliva in a matter of weeks or months, but if changes are made to the way it fits or it's made poorly to begin with, it can kickstart a new round or never stop. It's worth mentioning those things to the dentist. If he's skilled, he may be able to adjust whatever it is to stop the spitting.
People with no dental work or appliances... I don't know. I tend to think there's probably a split- with some, it may be a mechanical problem anyway. With others, perhaps their bodies produce more saliva. And, there's also a possibility that they just move their tongues in a way that causes the spray. In which case, I wouldn't call it a habit, but I bet a speech pathologist could help them relearn how to talk or give them exercises to practice that will reduce it. Bottom line- I don't think this is something an adult can correct on his or her own. They'd have to talk to a doctor or dentist to find out why it's happening, then find a solution for it.
Randy D 