The answer to this question lies inside of the famous 'prophecy'. 'The prophecy' of course refers to the prediction foretold by Sybil Trellawny in Hogsmeade pub in the year 1980. We know Sybill as the professor of divination at Hogwarts, and we know that she has a track record of successful predictions- predicting at the end of the 93-94 school year that Pettigrew would escape, and that Voldemort would return. However, she was not always so successful, and in 1980 found herself in the Hogsmeade pub with Dumbledore, interviewing for her post as professor. While her poor reputation preceded her, Dumbledore gave her a chance because she was the great grand-daughter of famous seer, Cassandra Trelawney. During her interview, a disappointed and unimpressed Dumbledore began to lose interest, when suddenly Sybill entered in to a trance. In her trance-like state, Sybill spoke of the famous prophecy that we all know, explaining that Voldemort would be defeated. It spoke of a boy, that would be born at the end of July, that possessed the power to defeat the dark lord himself. What Dumbledore and Sybill did not know, however, was that Snape (who was then a death eater), was eavesdropping at the door. This simple fact immediately placed Sybill in great danger from Voldemort- and in an effort to protect her, Dumbledore hired her for the position at Hogwarts.