Either the man himself sinned or his parents sinned. The disciples were well-acquainted with the following verses which seem to say that bad things are always a result of our sin. No harm happens to the righteous, but the wicked are filled with trouble. These verses depict God as a righteous judge who gives us exactly what we deserve. Life's tragedies are the result of sin.
Bad things happen as a result of our own sin.
But this approach doesn't always fit the facts. The Book of Job clearly shows that sometimes tragedy comes even to the righteous. A man born blind has not had the opportunity to sin. Bad things sometimes happen to good people. Since the man was born blind, the disciples expected Jesus to either say that his blindness was caused by the sins of the parents or else explain how a child can sin before it is born.