I think that, to some extent, psychedelic trips can "awaken" the mind to future experiences. Like LSD or psilocybin, THC seems to have some of these elements that cause an "awakening". While there may be no physical changes to the brain, you still "wake up" to new types of experiences that you did not know were possible before, and the pattern-recognition part of your brain tries to connect new thoughts to those memories. So when your brain enters a similar headspace to where you were during the original "episode," it will try to find similarities between those events (despite the absence of THC). This connection might be as simple as just listening to jokes, and your brain would connect the action of processing a joke to what you felt at the time of the high. Using its "learned experiences," your brain would try to apply the old thought-process to that moment. Then again, THC has been shown to have some persisting effects on memory and these are not fully understood yet. For all we know, it could be a long-lasting effect of THC. Either way, this sounds kind of similar to HPPD, and the idea that it could be a subconscious attempt to connect two events seems very possible as an explanation to HPPD in general.