The Hufflepuffs, who saw Draco grab Luna before she fell to the courtyard, started spreading the story around. At first, no one believed them, but when Luna nodded sagely before floating to class, it took hold. Maybe Draco Malfoy wasn't such a bad guy after all.
A couple of days later, the two Hufflepuffs showed up dishevelled and bruised in the Great Hall. They were tense and skittish and said Malfoy had threatened and beaten them to make them say they saw him save Luna. "I think he was trying to make everyone forget about last year."
Someone's reminder that Luna actually had corroborated the story was countered with what an utter shit Malfoy was, and there was no arguing that.