What causes frequent spitting in children?

Bad manners.

Depending on where and when plus age of child perhaps he/she has just discovered they can spit, which is usually around the age of 4 years, if the child's just found out they can do this ignore it and the child will stop doing this on their own, if you make something of it then you'll find the child will probably keep doing it, however this does all depend on the child's age.

Anything your child does and you make a fuss or something over it, they will usually do whatever it is more often, because it gets attention and children have no idea of attention being positive or negative.

Mainly if you just ignore it they will soon stop once they realise it no longer gets attention, Same applies if the child starts using swear words, take no notice.