I saw it as well. So Ohno touched the Canadian skater as he got close - normal move and normally not considered a reason to disqualify some one - but the Canadian skater fell. Since the skater fell, the ref declared that Ohno caused the fall resulting in the disqualification.
What makes it seem even worse is that the winning Canadian also "touched" the Korean skater and the Korean skater fell. But the Canadian was not disqualified, but instead won the gold.