While the early relationship between Christians and Jews no doubt played a major role in the history of the Greek versions, there was another factor that should not be overlooked. The recently discovered Dead Sea Scrolls provide indisputable evidence that at the turn of the era, before the birth of Christianity, the text of at least some books of the Hebrew Bible circulated in more than one form. One of these textual forms, however, emerged as the standard text by the beginning of the second century C.E., apparently supplanting all previous Hebrew texts. This situation alone could provide the need for a new Greek translation faithful to the newly standardized Hebrew Text.
from page 38, Invitation to the Septuagint