Why did juliet ask romeo pardon for what he had overheard in the garden?

She asks his pardon because she has been declaring her love for him out loud (and she didn't know he was there).

Basically, Juliet is embarrassed by the things she said out loud. He heard her deepest thoughts when she thought no one was listening, and she knows it's not good form to admit your love for someone five minutes after meeting them. She apologizes that her love seems quickly won - - she says that her love is true, regardless of how quickly she came to love him.