.
I think that if they were mixing substances, and then SS left and KR called and surprised SS, KR might have been in a dissociative, pre-ictal, not very clear, state. Which sometimes foretells a seizure. Drug-induced seizure is not unlikely. Especially with some substances, with stimulants. (Did she hit her head?)
The rest is with people who were there? Maybe they simply left when they saw her, moved to another place not to be responsible for all of it, drugs, minors. KR, being postictal and confused, decided to drive, or even walk, and in her confusion, something happened? Could the decision to "move to a harder-core party" be related to the fact that she had a seizure and they simply abandoned her?