Why did lady Capulet ask the nurse to leave then ask her to come back?

There are several possibilities. First, the nurse's story went on and on when Lady Capulet wanted to get to the point and talk about the possibility of Juliet marrying Paris. Secondly, the nurse is one of those annoying people who is not satisfied with telling a joke once; she has to repeat the punchline over and over again, with remarks about how funny she thought it was. The nurse repeats this joke three times: if I were Lady Capulet I'd want to strangle her.

Finally, and most importantly, it's a dirty joke. When Juliet was about two, apparently, she fell on her face and the Nurse's husband said to her (paraphrasing freely) "You may lie on your face now, but you'll lie on your back for men when you're older" upon which Juliet stopped crying and said "Yes". Not only must the Nurse's husband have been an odious and sleazy man to conduct a conversation of this kind with a two-year old (nowadays it might get your children taken away by the authorities), but this greasy and distasteful anecdote does not set the tone Lady Capulet was looking for when addressing the question "How stands your disposition to be married?"