Also, flashbacks are nothing like what people seem to imagine they are.
It's not caused by residual LSD, because LSD has an incredibly short half life, and if evacuated completely out of the body in less than a week.
A flashback is simply a memory of the experience. You might, for a couple of seconds, see the walls breathe slightly, or see sound, or have an unusual shift of consciousness, but that's only a result of being primed to the experience.