It's shocking to hear your child swear, even worse when it's directed at you. It helps to know where the swearing comes from. When kids hear adults swear it's always in an attention getting tone of voice.
Emotions are on high.
The air crackles with static. Kids try out cuss words to see if they can create the same atmosphere and get the same kind of imagined respect. Younger kids are just playing mynah bird. To get attention. Once a child uses a bad word and gets a startled and immediate response from the adults around him he realizes what a powerful tool it can be.