Keenan is having a flashback. this experience is most likely due to keenan's prior use of

The answer to this question is Hallucinogens

Hallicongens substance will disrupt the part of brain that produce serotonin and our nerve cells.

This disruption will often make the user unable to differentiate which one is fiction and which one is real, which will give the flash back effect to users such as Keenan