However, the other Slytherin’s believe she did it because Draco was really annoying and are inspired to do the same.

Lots of the former Slytherin’s get re-sorted into Ravenclaw or Hufflepuff instead.

Soon enough Draco is basically the only Slytherin left. Even Crabbe and Goyle got re-sorted, somehow getting into Ravenclaw. Pansy went into Hufflepuff, whilst Tracey and Astoria followed Daphne into Gryffindor.

(Daphne decided she had to be re-sorted as being in Slytherin meant that other Slytherin’s would try to interfere in her relationship with Harry)