The only drugs to give me "flashbacks" are salvia and speed- salvia remembering often hits me in uncertain situations, anything anxiety inducing. I can often achieve 'total recall' of my salvia trips when shocked suddenly. Amphetamines have haunted me through periods of stress and anxiety- those feeling are so closely related to the amphetamine high that its not surprising.
As people have said, its an anxiety/panic thing. Anyone that has experienced a sober panic attack will know how altered you feel, visually, mentally etc. After you've taken psychedelics perhaps your more inclined to notice these things and attibute them to the drug?
Basically i can have 'flashbacks' about any experience, good or bad, sober or tripping....its called memory. This myth seems to be centerterd around the misinformed notion that acid is stored in your spine, possibly leaking out and causing a short, spontaneous trip. As long as people are aware that the flashback experience has nothing to do with active chemical being re-released, thats fine.