.
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 lies with people who were there. My guess with this scenario would be, they simply abandoned her when they saw what was going on, rapidly 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 of the group to "move to a harder-core party" be related to the fact that she had a seizure and they simply abandoned her? Didn't hurt, but didn't help, either? Such a response to drug-related event at parties is not unusual.