Explanation:
During the quidditch match, Hermione notices how Snape is keeping his eyes locked onto Harry, while doing so he seems to be muttering things which leads her to suspect he's cursing Harry's broom.
Once she sets fire to his robes, it seems as the the "curse" on Harry's broom was broken, the events soon after that led up to Harry winning the match.
Ron and Hermione tell Harry that Snape was trying to curse him, so then they proceed to tell Hagrid who tells them he doesn't believe it.
The next piece of evidence they provide is how Snape was hurt by the guard dog that was guarding the "mysterious package."