Why does Cherry feel responsible for the events at the fountain?

Cherry feels guilty because of her interaction with Ponyboy and Johnny during and after the drive-in. Her refusal to get into the car with Bob and the other afterward was one of the causes of the event at the fountain.

From the text:

Dally was going on: "She said she felt that the whole mess was her fault, which it is, and that she'd keep up with what was comin' off with the Socs in the rumble and would testify that the Socs were drunk and looking for a fight and that you fought back in self-defense."